Probably soon. You'll notice her digging a few days before she lays. I've had leopard geckos take up to 4-5 weeks after being paired to lay their first clutches. After that it will probably happen every 2 weeks.
EDIT: I was tired this morning and forgot to add one thing. She looks very heavily gravid, so I think you will definitely have eggs within the next few days.
When should I do the eggs candling to check if the eggs are fertile? For now I'm mixing 1 part of perlite with 1 part of water for the incubation substrate.
Sometimes you can see they are fertile right away by looking for a pink "bullseye". Other ones are less obvious but you should notice a rosy pink color rather than yellow at about a week.
I think most people use 1 part perlite to 0.8 parts water. But I use vermiculite, so don't quote me on that. Maybe someone who uses perlite can chime in.