The right answer is 60 at the moment, as answered in:
This value is set by the protocol-dependent constant
Thus, it can eventually change on future versions of the protocol.
Notice also that 60 is an upper bound: the current implementation of the mempool performs a recycling process each time the node changes its head, and incompatible operations are discarded.
EDIT: This reply is outdated, as the adoption of the Granada protocol changed
max_operation_ttl to 120 blocks. However, notice that given the halving of the minimal time within blocks from 60 to 30 seconds, the window still has a similar length, when measured in human time units, i.e. ~1h.