Following severall websites Britney Spears mentioned in an interview she will continue to make music for at least ten more years. She wasn't especific but she said to a germany newspaper: "I'll give myself ten more years. I want to have more kids in the future, marry and get some movie roles. I haven't met the right guy but everything happens for a reason which is good. I have no regrets!"
I think the same too. She has said before that she goes crazy if she has nothing on her hands for too long and music is where she does the most, so she will always end up doing something.
Poor Beyonce, Shakira, Christina Aguilera, P!nk, etc. Britney isn't gonna let any of them snatch the #1 female artist of their generation She's here to stay.