TSMM was open for us last weekend, we arrived around 10am and the CM I spoke with said it would have soft openings on and off the next few weeks. Not sure how reliable that was but we waited roughly 5-10 minutes each time and had such good fun.
Toy Story Midway Mania officially opens on May 31. Until then the ride may open up from time to time. The only way to know for sure is to go to the attraction. Guest Relations, The Tip Board will not know, nor will other managers and castmembers in the park. The ride may be up and down throughout the days and the only people that will know will be the TSMM Castmembers. There may be days where it will not be open at all.