eAct=40000;(*Reasonable?*)
preExp=10^8;(*Reasonable?*)
temp=2000;
gasC=1.987;
k=preExp*Exp[-eAct/gasC/temp](*1/s*)

conv=0.99;
Solve[1-conv==Exp[-k*t],t]

lrr3@psu.edu (updated 10/08/2015)