Egg shells can provide some calcium but not enough so just supplement the way your doing. If she is young and just started laying she’ll probably lay steady for at least two years with breaks in that time also. Once they hit two years they can begin to slow down but they won’t stop completely. I think every breed is different my Muscovys are going on 7-8 and they just began laying for this season my Runners and Buffs laid straight through for 18 months when they first started laying. They are going on 3 and this year took 3 months off before I got an egg this morning I got a Buff egg . Main thing is good nourishment and exercise to keep them in good shape for laying and not to let them get over weight.