How long to do plan to breed this breed and is it going to be a closed flock or will you add new blood to it in the future? In the short term you wont have any problems at all. If not keeping records and having a few cock birds breed all the hens then 10ish years might be problem free. Longer than that if you didn't tag and keep records you could run into problems like lowered vitality and poor hatch rates. Not to mention your birds pyhsical attributes could be moving far from the standard for that breed.
People do keep closed flocks for multiple decades without issue. It's through planned line breeding or spiral breeding pens they can accomplish that. Basically, there is plan and you have records on everyone. You know who the father and mother and grandparents and so on are of every bird in your flock. In this way you maintain genetic diversity and improve the flock toward the standard of that breed.
Say record keeping is not your thing. Say you purchase hatching eggs from another breeder every 8 to 10 years to grow out and incorporate into your existing flock. How can you tell which is the new birds to your current flock after they grow out without color code bandettes or colored zip ties? See what I mean, you could with minimum record keeping and colored bandettes know a lot about your birds. It's doesn't have to be serious accounting if you don't want it to be.
Otherwise, yeah. You can let your flock go and hatch eggs for as long as you can. There will be a time when hatch rates suffer due to fertility and/or vitality issues. When/if this happens get another cock bird or hatching eggs to bring in. The infusion of that new "blood" will bring your vitality back.
Most of the time people change breeds or rooster before inbreeding becomes a problem. If your not breeding to improve genetics,and only breeding for eggs I would not worry about it. Most All chickens were inbred at some point. Anyone who tells you different, is lying to you.
I'd not worry about it for a number of years. Sometimes your plans change... sometimes your flock dynamics change unexpectedly. Unless you have a problem that shows up in your flock: a particular abnormality or weakness, just keep on keeping on. Cull the weakest birds from your flock, and hatch your best eggs, and you should see a general improvement over the years. If you start to see a decline in health, or fertility, or quality of bird or egg, then you could bring in new blood.