The short answer is often. The American Cleaning Institute recommends you wash your PJs after three or four wears, noting that if you shower before you go to bed, you can get away with not washing them for a couple of more nights. People shed dead skin cells while they sleep in addition to sweating, and it all ends up in your pajamas, bedsheets, and mattress, so it's important to maintain a hygienic sleeping environment by washing these frequently. If you tend to lounge in your PJs for more extended periods during the day or if you step outside in them, you should wash them even more often.