Normally 22 - 34 days, but that's just a guide I've seen around online. My first female gave birth around 35-36 days, and my last female guppy gave birth at 19 days o.0
The advice I was given, is just basicly keep an eye on her. She will continue to grow and before birth, you will see her staying in one spot for quite awhile and arching her back. LoL You can also tell by the little fry popping out of her

Just keep a close eye on her. There's nothing more you can do