How long does mortar joints need to cure before painting?

There are so many factors that go into the curing of concrete. Ambient temp, ground temperature, wind, rain, sun and many more. A good rule of thumb is wait a month. Around the month time frame place a square piece of plastic sheeting (say 6X6 inch piece). Tape all 4 sides with Duct Tape. After a day or so if you see condensation under the plastic the concrete is still too wet to paint. Keep checking until it the plastic is dry.