Can someone tell me how long will it take for a foot blister to heal without trying anything?

SImple means: You need to identify why the blister developed and eliminate that problem. Then, the keys are to cover and protect the injured area with dressing changes every other day to allow healing. Remember that inflammation (red, hot, swollen and tender) are important parts of healing and do not necessarily mean infection. If the wound is healing, it cannot be infected. If it gets worse, see a doctor.