Most university regulations ought to have some sort of timeline within which the viva should be held following submission. Given the stats on viva outcomes, where the vast majority are a pass with minor or major corrections, you might want to factor that into your timescales...you could have a viva in May, and then be given six months to do major corrections....my own personal suggestion would be just to focus on the quality of your thesis and submit when you and your supervisor agree its ready, not just targetting the graduation ceremony...most universities have a winter ceremony as well. You want to make sure you get to graduate, be it summer or winter. But as to timescales, check your university regs, those likely have information on when the viva is to be held after thesis submission, and timescales for corrections.