General rule is 7-10 days. Once you know how a particular strain handles nutes, you can do top ups indefinitely if you want. I've run straight through beginning of veg to harvest just doing top ups when the res gets down to 50%, and I'll let it go almost dry every 2-3 weeks and do a 80%+refill. As long as your not overdoing your nutes and keep a schedule that slowly increases ec then tapers off at the end you shouldn't have any issues. I like it because I can control ec rates better, rather than go 1.4 one week to 1.6 the next, I can go 1.4, top up at day 2 to 1.45, top up at day 4 to 1.5 top up at day 6 to 1.6, etc, etc. Same with the Ec drop at the end.