A difficult question to answer judging from the many, many discussions about incubator humidity to be found on just this board. For the first eighteen days (for chickens) the books and extension documents say the humidity should be in the 50-55% range. BUT we have a number of folks who incubate at much lower humidities who have good success rates. And at least one fellow who hatches in the high desert (low air pressure, very low humidity) who hatches as I recall at a humidity of 70+% or better.
The last three days (when you stop turning them) you want to bump the humidity to at least 60%. I favor 65% and some go higher.
The bottom line is that by day eighteen the egg needs to lose roughly 12% of its mass in water weight. How much humidity is too much depends on your altitude, local humidity, and your incubator. If it were me I'd stay with the levels indicated in the incubator manufactuerer's instructions until you have good reason to know better. If you don't have any instructions then keep it in the 50-55% range for days 1-18 then bump it to 65% for days 19 through hatch. Experience will tell you from there whether that was too much or too little for any successive hatches.