How many Windows handles in use is "too many"?

The desktop heap, which is a pool of memory where the real "stuff" the handle represents lives. It's sometimes not so much how many handles you have allocated but how much memory each object under that handle is using. You can debug the heap this way. It is a pain to install.

(this was recycled from another one of my answers)

See Raymond Chen's post on this topic. The window manager enforces a limit of 10K per process, and has a total limit of 32K across the system. So if it "only" leaks 100 handles per hour, then you have a few days of uptime before it starts misbehaving.

Note that not all handles are equal. Window handles are not DB handles, for example, and may follow different rules. So this restriction might not apply, depending on what sort of handles the program is leaking. Also read this blog post.