There is no 30 day limit, she can enter at any time after the day that the visa becomes valid. If she leaves it too long she might face trouble when she applies for ILR/probationary citizenship, however spousal visas are issued for 27 months now, so as long as she enters within the first three months of validity she shouldn't have any trouble. Although, visas can be post-dated for up to three months, so the smartest thing to do would be to decide when she wants to travel and have the visa issued beginning that day.