I am thinking about getting a t-rex for flying in the park when i can't fly nitro. How much flight time does a1200, 1500, 1900, 4200mA lipo last on a range of t-rex models?

Most people run LiPos in the 2000-2100mAh range (assuming 3s packs that the stock motors work with)
Your 1200 and 1500 will probably not handle the current demand. Your 4200 will probably be heavier than is desirable.
With my well used Thunder Power 2100's, I am getting 8 minutes. Less than when new, but also I am not pushing for the longest flight time. I have several packs, and I think they will last longer if I do not run them down 100%.

Vince

Try the Flight Power EVO Lite 2500 I've been very impressed so far.

Trex450 SA- every thing stock, jgf450th, Jazz, EVO1800 25C gives 7 minutes putting back 1300maH (less than 80%). AUW 810 gms, H/S 2500, 325 pro wood blades.
Wb