When to plan maternity leave?

Sandra
I'm about 12 weeks along and I'm know it's alittle early but when should I start doing stuff for maternity leave? I'm due May 15th and I get 4 weeks before the baby is born so my plan is to go out early April and come back early July. I work in retail so I'm missing most of the "100 days of hell". When should I start doing paperwork and stuff?