Donrae got it right as usual.  It takes an egg about 25 hours to go through the hen’s internal egg laying factory.  That egg can only be fertilized during the first few minutes of that journey.  So if the mating took place on a Monday, Monday‘s egg will not be fertilized, Tuesday’s egg is questionable, Wednesday’s egg is almost certainly fertile.  That’s after a mating.  Not all roosters mate with all hens in their flock every day so it may take a bit longer than that for all eggs to be fertile, but it usually doesn’t take long.
The hen stores the sperm in a special tube very near where the egg starts its journey.  The sperm might possibly stay viable for over three weeks but not always.  The older it gets the less viable it is.  Most people use two weeks after a mating for how long the sperm stays viable.  
I’m making a lot of assumptions from your post.  It’s quite possible I have some things wrong about your situation or that you already know a lot of this.
How old are your chickens now?  I suspect what you saw was hormones out of control during adolescence.  The boys normally mature earlier than the girls, are bigger and stronger, and have no parental controls on them.  The girls normally mature later than the boys and have no idea what’s going on so they don’t know how to play their part.  The mating ritual is not just about fertilizing eggs either, it’s also a dominance thing.  The one on bottom is accepting the dominance of the one on top whether willingly or by force.  When young chickens go through puberty like that it can get pretty wild.  But once they mature and take on their adult roles, things normally calm down a lot.  You might find that if you put one rooster in with your 6 hens, things are not all that bad.  
A lot of that depends on the age and maturity of your chickens, male and female.  Some hens will squat for practically anything in spurs but a mature hen often expects much more from her mate and potential father of her children.  He should find her food, dance for her, keep peace in the flock, and watch for dangers.  He also needs to WOW! her with his magnificence and maturity.  The male has to earn certain rights and privileges.  Adolescents often don’t have the maturity to do that.
You occasionally see posts on here where “The rooster is raping my hen!!!”  Actually he’s not.  A normal mating should go something like:
The rooster dances, lowers his wing and sort of circles the hen – He is signaling his intentions.
The hen squats – This puts her body on the ground and transfers the rooster’s weight through her entire body into the ground instead of going through her legs.    This allows the hen to support a much heavier rooster without damage.
The rooster hops on and grabs the back of her head – This head grab not only helps the rooster keep his balance and get in the right position to hit  the target, it is the hen’s signal to lift her tail out of the way.
The rooster touches vents and hops off – His part is done.
The hen stands up, fluffs up, and shakes – This puts the sperm in the right storage container.
Not all matings go this way.  Often it involves some running away and chasing.  As long as the hen eventually squats and the rooster does not draw blood, it is pretty normal.  Some hens never willingly submit to a rooster, some roosters never learn proper technique, but with maturity practically all do.  It’s normally when the males are still immature you see the worst of these.
There is no magic number concerning a rooster to hen ratio.  Many breeders keep one rooster penned with one or two hens throughout the breeding season and don’t see rooster brutality to the hen, over-mating, anything like that.  But the secret is that they use mature chickens.  They let them mature so they can see if they fit their breeding program.  I still recommend you keep as few roosters with your hens as you can and still meet your goals.  That’s not because you are guaranteed problems with low ratios but because the more roosters you have the more likely it is that there will be problems.  That’s part of the reasons I suggest you only put one of your roosters with the hens.
Part of that too is I don’t know how much room you have.  As long as they are kept in their separate bachelor pad, the two roosters will probably get along fine.  But if you mix them with a hen, they will almost certainly fight for flock dominance.  If you have sufficient space these fights usually end with the two roosters working out an accommodation in how they will work together to protect the flock.  But for them to work out that accommodation, the losing rooster has to run away from the winner.  Some chasing will be involved.  Also, they two often split the flock and go their separate ways during much of the day to help avoid conflict though they will occasionally intermingle and can share a coop and roost at night.  If they don’t have sufficient room to run away and avoid, these fights are more likely to end badly for one of them.
Donrae’s suggestion to put one rooster in with the hens and pull him back out after a few days would work.  You can even alternate the roosters if you desire.  You’re likely to see some skirmishing when you put the two roosters back together, but you’d probably see the same type of stuff if you separated two hens and then put them back together.  They have to establish the pecking order.  But you may see that the rooster and hens get along so well after a day or two of getting acquainted that you decide to keep one rooster permanently locked up or even get rid of one of those roosters.  A lot of that depends on your individual goals and desires.
Good luck!   Your goal is certainly doable.