How long is hard to say, since every snake grows differently. Probably a good while though. The best general rule to follow would be...
Length of container + Width of container = Length of snake
or
Length of container = 2/3 Length of snake
Approximations work, and those are for minimum sizes, so all I could really say is your container would work until the snake outgrew those sizes. Which could be quite a while, or a few months. Again, all snakes grow differently. If I was to give a rough estimate based on the approximate dimensions and average growth rates it could probably easily last till about 6-8 months. Don't take that as a certainty though, just an estimated guess.