Depends totally on the contract at that major. Some have long call on reserve that gives you up to 12-18 hours to show up for a flight. Some want you at the briefing 2 and 1/2 hours after they call you. What does the contract read for whatever major you are looking for and how sure are you that you will actually be awarded your base of choice in a reasonable timeframe? And if/when you upgrade, will you still hold it as a Captain?