Based on how you’re describing where your business is at right now, I would say a much more important question is WHAT marketing to do, rather than how much of turnover to spend (though to answer your question; 5-15% of turnover, depending on what stage you are in your business). I would also look at the difference between marketing and advertising.
In the early days, we tried almost every form of marketing/advertising under the sun; flyers in letterboxes, ads in the classifieds, Adwords, FB ads, newspaper inserts, article write ups, backlinks from allied sources, influencer marketing etc.
Ultimately, this experimentation allowed us to see what worked and what didn’t, and it really varies so much depending on your industry, your niche, your ideal market/client.
We have gone from spending $1,000 a week on advertising to close to $0, yet we are ‘marketing’ constantly. Our ideal form of marketing has turned out to be referrals, good organic search rankings, reviews and testimonials from existing clients and partnerships with other businesses, where there is cross-promotion. We put a lot of time and energy on marketing but we don’t actually spend money on advertising per se. If your question is motivated by wanting to see a better ROI, definitely investigate the marketing route over the advertising route.
Also, try ‘This is Marketing’ by Seth Godin for a bit of guidance 
