How long a painting should take you to do is impossible to say. It depends on every individual artist, their technical painting skills, and what they visualize the painting to be.
Some famous artists have taken months and even years to finish a painting. The 19th-century French artist Ernest Meissonier took 13 years to finish his painting Napoleon's victory at Friedland which is 53 1/2 x 95 1/2 in (136 x 242.5 cm) in size.
If you fuss with a painting for too long, you run the risk over overworking it. If you declare a painting finished too soon, you run the risk of not developing the idea to its full potential. If in doubt about whether you should stop or continue with a particular painting, you should consider creating another version of the painting, or creating a series on the subject.
Ultimately it's not about how long a painting takes, but about how pleased you are with the result.
