Converting H4 to F1

[trt987]

My wife is on H4 visa currently and is also doing her MS from USA. As my Green Card will take atleast 2 to 3 years to come, we were thinking of converting her H4 to F1 so that she can work once her MS finishes.
Are there any issues wit this?
How long does the F1 visa take to come?

Thanx
gaurav