Hey, female Phelsuma can/will start to lose their calcium sacs as the breeding season carries on. As far as Phelsuma go, with a lot of the species you can see the eggs through their skin. I have not bred P grandis though so I'm not sure if it is common to be able to see their eggs.
As far as combating the lack of calcium you can try supplementing extra. I am using Repashy Calcium Plus. It has lower levels of a couple of the goodies that can be borderline toxic in other supplements. I hope someone can post some good links about this- here is one
So confused about Repashy Calcium Plus.. - Page 2 - GeckoForums.net
As far as frequency goes it can vary some. My P laticauda has laid eggs with as little as 23 days inbetween sets and much longer between. If all the husbandry requirements for them is met they will lay and lay and lay. One thing you can do is seperate your pair. This can slow down egg laying. At the same time, the female will lay eggs regardless of being mated or not. If she lays dud eggs sometimes they will eat them(getting some calcium back!)
Keeping the geckos as a pair doesn't really leave all the options to each animal. Phelsuma will fight too if things aren't going well. My P pasteuri don't like each other much at all when the girl is getting ready to lay eggs. On top of this, I have to give them 2 feeding dishes of the Repashy Day gecko MRP because they fight over the dish(Even though there is more than they can eat in 3 days in the cup)
You'll have to decide if your geckos are getting along and if the eggs she is laying still look "good". If either doesn't seem right I'd seperate them and give her some recovery time.