If your battery goes flat in 3 weeks, you need to find out why. See if you have a light staying on or something somewhere that's pulling power. On a modern vehicle with computers and other gizmos that always suck power you should be able to get a couple months of sitting before even worrying about the battery charge state. Older cars without any ECU's or parasitic draws could go for several months.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]