From the Chicago Tribunes daily prior watch
Status:Ready to take the next big step.
Return Target date:Late May/early June
Outlook:Prior took Sunday off and isn't expected to throw off a mound again until Tuesday in Phoenix. Minor league pitching coordinator Lester Strode will monitor Prior's rehab at extended spring training when the team leaves Arizona after Wed. game vs the Diamondbacks. The rehab program already is in place. "They'll have the full program, and I'll get the details dail," pitching coach Larry Rothschild said.
Hope I could be of some help. I be pessimistic and say early June, since the Cubs want him at 100 percent.