Generally speaking, such transfers must vest before the end of the maximum period, or the grant will be void. Under the old common law, a transaction would be void even if the property might possibly vest after the end of the maximum period, but now most jurisdictions have, by statute, adopted "wait and see" laws.
In essence, transfers of this nature typically need to take effect before the end of the designated timeframe, or they will be deemed ineffective. Previously, under common law, a transaction would be considered null even if the property might potentially vest after the deadline, but most jurisdictions have since adopted "wait and see" laws through legislation.