That would be a function of:
a Original size of the routing table.
b. Routing protocol in use.
c. Position of the router in the hierarchy (core,access, distribution).
d. Processing power of the router, in conjunction with its work load at the time you did the clear route.
I doubt anyone can come up with a hard and fast number. Way too many variables involved. I would suggest experimenting
if possible with a non production device.