Just me, personally...

I generally tip $2 per night (or equivilent in local currency) in most instances.
I usually go $5 for a suite or if the housekeeping service goes or has to go beyond the normal line of duty. On the flip side, if there's an cleanliness or service issue during my stay directly relating to housekeeping, I don't tip at all.

(I'm also a guy who routinely goes 20% or more in restaurants, but also a couple of times a year will write "not aplicable" in the "gratuity" line of a restaurant bill if the service is lousy).

Again, this is just me. I'm not trying to suggest anything to the next person.