Swimming is an excellent way of exercising and keeping fit.

As far as timing is concerned the answer is: whenever your wife feels ready.

Chemotherapy affects different people differently. With any treatment regime some people will have more problems than others and the time it takes to get over these will vary from person to person. For this reason it is difficult to give precise time but if your wife feels well enough to go swimming and would like to do this then it is perfectly all right to restart. It would be a good idea for her to take things gently at first and build up gradually to the level of activity that she managed before her treatment. This is because even if she feels well it is likely that the chemotherapy will have drained some of her stamina and she could find she tires quite easily. This is nothing to worry about and full energy levels will gradually return in the months after treatment.

The one word of caution is if your wife has had a very low white blood count at times during her chemotherapy (which can sometimes happen as a side effect of treatment), then she should check with her doctors or nurses that the blood has recovered to a safe level. This is to ensure there is no risk of infection in going back to a public (and possibly crowded) place like the swimming baths.