I think differently about this, because of two reasons:

1) I've always (and only) been involved in bootstrapped startups; and

2) I've been lucky that those startups grew organically and fast (enough) which minimized our need on marketing spend.

Instead of deciding on a specific budget for this, I would instead look at your current priorities (in terms of budgeting and re-investment into your team):

1. Build a great team.
2. Build a great product.
3. Craft incredible customer experiences.
4. Spend money on marketing.

If you've already hit all 3 top priorities and you can't reinvest any further into those, then you should start spending money on marketing.

If you don't have revenues today and you are hoping to generate revenues through marketing spend, you're on slippery slope (says the bootstrapper). Whilst not wrong, this is tricky and you'd need to take a realistic look at your customer acquisition cost (CAC) and how much you can invest into acquiring new customers.