A high level place to start, is to go and explore the recent financials of SAAS companies that are public. You can extrapolate. From their spend and rate of growth, a few examples that can inform how much you should spend on marketing. This then should be your starting point. Then step back and look at your marketing objectives, your acquisition costs and LTV. And work backwards to see if this is going to drive the results you need.
If it isn’t, then you need to make some adjustments. Whether it’s more budget, or other investments to help improve your acquisition cost. This process will help you get to a tailored level of investment for your own objectives, informed by what the market is spending.