If you have an Eruv Chatzeros for carrying, how often is it necessary to check it to make sure that it is properly strung and you can still carry on Shabbos?
Ideally, you should check the eruv before every shabbat/day you make use of it.
However, there are those who say that if there is no major weather change you can rely on a chazakah. But I believe this is only for eruvin that are hard to check. You really should check every Thursday night or Friday before Shabbat.
If it rains or there is a strong wind or snow during shabbat most say you can still use it.
I welcome sources from the community. (My source is from being on an eruv checking committee for a few months)
I think you need to check any individual string, lechi etc. more often then it would break on average because then you have a chazaka.