There's obviously some happy middle ground between a year or two and unlimited terms, but there are problems with both extremes.
A 2-term limit on a position with a 2-year term can be pretty hard. A 2-term limit on a 4-year term is a different matter.
It also shifts expertise out of the legislator's staff and into the lobbying apparatus.
If the term-limits are brief, then lawmakers have to frequently rely on the party's election and fundraising apparatus, and therefore feel pressure to tow the party line.
If the term-limits are extended, then lawmakers aren't as beholden to the populous, and can therefore become corrupt.
I think I first read this balance articulated (much more clearly than I'm putting it here) in a book by the economist James Buchanan.
For Congress/Parliament members, perhaps a little longer would be reasonable, but probably no more than 20 years.
Good point.