How long is a piece of string? This is going to depend on how sophisticated your users are, how sophisticated your application is, what your users' expectation for handholding is, and how optimized your processes are for dealing with user issues before they get to your inbox.
The great thing about words like "active user" is that they mean whatever you want them to mean. For certain definitions of active user such as "Has downloaded the trial version of my software recently", I have a hundred thousand of them. (For very strict definitions of active user, ~2,000 or so.)
My support burden averages out to well under an hour a week.
On a very bad day, it is four emails, three of which will be about transactional issues ("I paid but didn't get my software!" "Begging your pardon, ma'am, but the payment processor doesn't tell me about it if you don't hit the confirm button on your confirmation page." "Oh thx!"), forgotten passwords, and lost Registration Keys, and that will take twenty minutes. An average day sees checks stats 1.2 emails. Many days have no emails at all.
It has not been my experience that support scales linearly with user count. It is closer to logarithmic but with major decreases when you take affirmative steps to resolve the 20% of the issues that take 80% of your time.