Hello,
it depends if you use BGP with the provider or not
a) BGP
use
sh ip bgp summary it will tell you how long is the session up
sh ip bgp neighbor can tell you from the number of sessions dropped how many times has gone down
to see the down times you need to look in the log file
sh log | inc BGP
b) if not using BGP only static routes:
you need to look in the log file how many times the wan interface has gone down
use sh log
note: if logging is not enabled you need to enable it
logging buffer 100000
logging buffer notification
Hope to help
Giuseppe