SJPP is a lovely city, but you can walk from one end of the main street to the other in ten minutes. I actually spent two days there, bcs I was nervous about a tingle in my knee and it is a long way to Roncesvalles. I washed my clothes, went to Mass in Notre Dame du Bout du Pont, visited the imposing Citadel, walked along the river. It was fine but at the end I felt that a night may have been enough. Particularly in the morning there are not many pilgrims, and it felt a bit odd and lonely.
Incidentally, the albergue I chose (Paroissial Kaserna) did not allow staying by two nights, except for an emergency, and I considered that I really did not qualify. So, if you stay two days, make sure this is not a problem.
Buen camino!