How often to Bathe.

With today's dog shampoos, it is possible to bathe your Papillon once/per week without drying out his coat.

Remember to brush him before the bath, or all mats and tangles are going to be far worth after the bath, when the hair got wet.
Fold his ears down, so the water does not get into the ear canal. Make sure to rinse his hair out very well, and apply a bit of conditioner, for easier brushing.
Have a dog hair dryer ready. There are some small ones available which come with a stand. Place the dryer about 1 foot away, not to burn his skin, and start with a low to medium setting to get your Papi used to the blow dryer.
This is the best time to clip his soften nails. (if you know how to do that)