The lengt of the useragent info depends on the useragent the client uses, and the info it submits. I just had a robot on my forum, and it had a useragent info of approx. 80 chars.
So I guess anything between 80 and 100 would be a nice number...
[edit]
here's the useragent info I'm talking about
:
sitecheck.internetseer.com (For more info see:
http://sitecheck.internetseer.com)
that's 81 chars...
[high]* Broekie sticks to the number 100

[/high]
[/edit]
[second edit]
just had one with 86 chars
[/second edit]