Is March of pleasant weather to visit Kunming? How long to stay in city and how many days outside?

Yes, March is a good time to visit KM.
For staying in the city, you may need about two to three days for traveling.
Outside, it is hard to tell how many days you can spend. If you just plan to go to the Stone Forest, you may only need one day. If you plan to go to Lijiang and Shangri-la area, you may need about a week to enjoy the beauty lesuirely.