You can pop the blister immediately, however, it's important to do so in a sterile environment. First, make sure you wash your foot thoroughly prior to lancing the blister. Then using a sterilized needle, make a small opening in the top of the tense blister. This will relieve some of the fluid and make the blister less tender.
Popping the blister will decompress the fluid in that area and make it less painful for you. However, one of the biggest mistakes patients make is removing the top of the blister. Please don't do this. The blister roof is the best biologic dressing that we have for this denuded skin. So please, leave it intact and don't peel it off!