Pretty straightforward.
assuming the linux box is connected to ISPA through eth0 and ISPB through eth1
route add default gw ISPAIPADDR dev eth0
route add default gw ISPBIPADDR dev eth1
this will route everything through eth0 until it goes down and then switch to eth1.
The time taken for the kernel to declare a route inactive is, by default 300 seconds. You can modify this to , for example 60 seconds by modifying the gc_timeout in the routes subsystem
i.e.
echo 60 > /proc/sys/net/ipv4/route/gc_timeout