Most managers will take between 10% and 20% of an artist’s gross income – including record royalties, publishing income, and touring and merchandising income. There will sometimes be a “sunset clause” i.e. a declining scale of payments due to the manager over a few years should you decide to fire or part ways with him (or her).
These are negotiable points, and many nuances and technicalities are involved. You should always have an experienced music business attorney (not your uncle Bob, the divorce attorney) review any management contract.