I've had limited experience with this. I worked with a ASP.NET MVP that was consulting with us. It was generally for half days and only about one or two times a week. It worked very well as I was able to absorb a tremendous amount of transfer and as any programmer knows mental block can occur at any time. It seemed that if one of us got stuck the other was able to jump in and bang out some code.

I think that used as described above for senior to junior knowledge transfer or for two strong coders working together to tackle a difficult piece of code pair programming is very useful. As any DBA would answer how much each week would be useful... it depends!