Thin uterine lining

I'm on my 7th cycle of clomid (3rd with IUI) and during my follicle scan my Dr. Mentioned my uterine lining might be a little thin (5mm).  I know this is a side effect of clomid that can occur after multiple cycles.  He is going to check it before my IUI tomorrow and if it less than 7.5mm he will give me estrogen.  Anyone here have that happen?  How long did you have to take estrogen for?  What form of estrogen was prescribed?