Dogs, whether Jack Russells or not, should only be washed when it’s necessary.
That means you should only wash them when they’re genuinely dirty and have been unable to wash themselves by normal tongue-grooming, or when they’re covered with a harmful substance, or if they simply stink more than you find acceptable.
Even then, you should use either pH neutral baby shampoo which also will not sting their eyes (e.g. Johnson’s Baby Shampoo), or special canine shampoo which costs a lot more but can also contain a flea-control medication.
Many dog-owners believe in washing their dogs more frequently, typically once monthly, and even if that is strictly not necessary it does have the advantage of conditioning your dog to accept being washed so that when it genuinely does need doing they will be accept it easily. Dogs washed infrequently often resist due to fear of the unfamiliar process and smells. I find that if I wash them regularly only as puppies they accept it when necessary later on