How much should I spend on Marketing?
Is there a certain percentage I should be spending of my profit?
This is a depend...
If you are spending money on your marketing and you are not getting a ROI Stop!
If you are spending money on your marketing and getting a high ROI - spend as much as you can.
It's all relative.
There is a school of thought that suggests 10% of gross (projected) for a start up reducing to around 6% of actual after you have reached critical mass (say by year 3).
However, I have also seen sound business cases based on double these amounts, so it depends on your competitive environment, market maturity plus the type of business you are in as well as your marketing operatives over time.