If there are 2 links between 2 routers (1 Ethernet+1 DSL), and I force OSPF to prefer the ethernet connection via giving the SHDSL link a higher cost - What is the worse case scenario (In seconds) that the redundant SHDSL link will be used in the event of the ethernet link going down?
Would it be the dead-interval?
As you can see below, you can set the fast hellos multiplier to anything between 3 - 20.
R2(config-if)#ip ospf dead-interval minimal hello-multiplier ?
<3-20> Number of Hellos sent within 1 second
However, you want to be conservative and set the multiplier to a high # as you mayn't want to see the neighbor considered down on non-receipt of 2 or 3 hellos. Performance shouldn't be affected much as hello packets themselves are very small in size and shouldn't consume much bandwidth.