It is a good idea every once in awhile to clear out the sms because it starts to drag the phone down. This is because it has to load up weeks or however many messages you have work of text messages and it begins to make the phones response slower
There is a limit on time. Standard is 30 days but you can change to 60, 90, and no limit if I remember correctly. With no limit you would be constrained by phone limit. SMS are small in terms of space required so I doubt you would run into memory issues very quickly.
Posted from my CrackBerry at wapforums.crackberry.com
I don't think there is a limit really. I have my inbox set to keep messages for 30 days and I never delete anything. I've never experienced the phone lagging due to there being too many messages in there. I'm sure there is a limit at some point (there has to be), but I doubt that it's something you would hit during normal use.
Posted from my CrackBerry at wapforums.crackberry.com