Sandy,
Marketo has a great slide on this. At least they used to do something like this:
New Lead: 1 hr to call
After 3 hrs, email to manager
After 24 hrs, email to manager's manager.
After 3 days,
After 7 days, CEO.
If I were running Sales, leads would have to be called back in 1 hr if Fast Tracked or attaining MQL. Failure to do this is very bad.
Other firms I've seen do:
MQL: 1 day
SAL: 7 days
SQL: 30 days to get to Opp
Opp: 30-90 days (depends)
Some firms do SLAs on Opp stages too. Not sure I would go that far except in very specific sales situations where you can close people with that kind of pressure.
Of course, you need to ensure vacations are accounted for properly.