There are two popular ways to set a marketing budget. One by percentage of turnover, the other by the likely cost of the task in hand.
Most companies allocate between 5 and 10% of gross turnover. If marketing is responsible for all your business growth (for example if you have no retail stores, direct sales force or telesales) then this figure may be higher. If you're going for higher market share, then the figure needs to be higher.
Some companies base marketing budgets on what was spent before and achieved before. If growth is needed, then so is investment in this area.