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?
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.
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)