Grimjaww said it would take ~2 hours or so for Ulquiorra to break out, starting from right before he started fighting Ichigo. Ichigo's fight can't have taken two hours (especially since he hadhis mask on almost the whole time, I'd say more along the lines of 10-15 minutes. Then as soon as that finished Nnoritra showed up and raped Ichigo for probably a good 5 minutes beforeell powered up, say another 5-10 minutes there for her to kick ass, then she goes poof and we get ~5 minutes of Ichigo getting raped some more by the other guy until Zaraki shows up, that puts us at around half an hour since Ulqiorra got sent away, there's still plenty that could happen before he should be coming back (assuming Grimmjow accurately measured his capacity, which is obviously not guaranteed). I won't be surprised if he shows up sooner since it would be written off as a misjudgment on Grimm's part, but I don't actually expect him to actually get out until a lot more has happened.