I'd say 6-12" would be reasonable.

The concern underlying the thread is of course overlapping wheels. As Event Services points out, working on being able to deal with overlapped wheels is a great skill to learn, and will give you confidence riding closer.

If you haven't done it, get with a friend or 2 in a grass field, long sleeves and long pants, and practice intentionally rubbing wheels. The trick is to pop off the wheel with a little countersteering. What normally happens when you overlap wheels is you're laying across the back wheel for a second, when it clears, you're still leaning over, and when the support is gone you go down, so you have to learn to pop off the wheel before that happens.