I currently run three campaigns, but the timetables for sessions are different. (I'm not insane or unemployed)
I have a semi-monthly canon-broken Eberron campaign that has been running for over a dozen sessions now and I'm about to pass the reins on to my girlfriend.
In my 'core group' we swap out DMs, systems and settings every month. Basically, a DM runs about a month's worth of weekly sessions then stops and the next DM takes his or her turn running. It allows DMs to plot and prepare comfortably without campaigns grinding to a stand still. My girlfriend runs a homebrew 4e world, another guy runs us through Shadowrun, set in the Philippines and I run Dark Sun.
I also have a 'rotating lineup' campaign set in the Chaos Scar, which I try to run twice a month. The idea is it's a 'campaign must go on so long as there are players' setup where I recruit at least four players from a large pool of gaming friends, level differences and party roles be damned.
Two of them run for about four sessions every three months, another one is weekly and the otdiffereher one is a short term one.