...
There is no need for the IKE and IPsec lifetimes to be the same at both ends. Rekeying is initiated shortly before the negotiated lifetime expires, usually after the shorter of the two routers’ lifetimes. However, under certain circumstances the connection may be lost during rekeying. If this is the case, it may be worthwhile to increase the lifetimes so that disconnections occur less often. This does require the lifetimes on both routers to have the same or at least a very similar values.
For security reasons, the lifetimes should not be too long, otherwise the keys could be compromised. Equally, the lifetimes should not be too short in order to avoid frequent and time-consuming rekeying.
...