Sonnanstine continues to show that he'd make a better reliever than starter, so his summer home should be in the bullpen where he apparently belongs.
For several years Joe Maddon, when asked any question in regard to Sonny, generally responds with a canned response about how Sonny warms up very quickly, which obviously points toward Sonny being a viable option for the 'pen.
The bigger and more decisive sign that points Sonny toward the bullpen is that his entire pitching career is based on deception. He doesn't have the capacity to blow heaters by batters, and doesn't have any truly filthy pitches, but he does have a variety of deliveries/arm angles and a huge array of different pitches, which enables him to be very effective the first (and sometimes second) time through the lineup. The Achilles' heel to this technique is that after seeing enough pitches from Sonny batters stop being deceived and start spraying balls all over the park.
Check out this chart if you don't want to take my word for it:
1st time thru LnUp
2nd time thru LnUp
3rd time thru LnUp
This alone should park Sonny in the 'pen as a mid-range to long reliever or even a possible set-up guy before someone who'll just pump fastballs by the batters (ie Balfour).
When do I think he should move in to his new summer home?
As soon as Kaz comes back into the rotation, obviously.
When (hopefully it's not and "if") Kazmir returns from the DL/rehab assignment someone from the rotation has got to go, and although Niemann hasn't been all that great, Sonny's situation would make him the more likely candidate to head to the 'pen.