is 5 months too short to ask for a recommendation letter

I have been working at my current job for 5 months and need my supervisor to write me a letter as part of the application process for a scholarship. Do you think 5 months is too short?
No, plenty of time for them to have seen that you are a good employee and are deserving of a scholarship.

Before you ask them to write you a letter of recommendation, always ask them if they will be able to write you a GOOD letter of recommendation! If they answer yes, then you have the green-light to request one, and you know it will be good.