Hello Ignacio
Note that the holddown timer is a MINIMUM. This means that if it is set to 90, then for 90 seconds, the new exit MUST be used BEFORE any new alternative exit can be selected. If the 90 seconds are up, and there is not yet any new available exit, the current exit will still be used. It does not go out of policy. In other words, for 90 seconds, the exit is locked in and cannot be changed. After the 90 seconds, it can possibly, but not necessarily, be changed.
Alternatively, the backoff timer is the amount of time a router will wait after a route is out of policy, before attempting to find another exit path.
I hope this has been helpful!
Laz