Running Multiple Websites?

[PFO]

For those of you who run multiple websites, at what point do stop focusing on one site and moving towards building another?

Do you filter some of the smaller websites to the larger one? Say you have a forum on your big site, would you have forum links on your other sites that would direct them to your main forum site?

I guess at some point you have to decide if it is worth investing more time in money hoping to see increased profits, or better for that time and money to go towards a new site. Just looking for some ideas on running and managing multiple sites.

Thanks!