How much should freelancers set aside for taxes?
There's no single number: it depends on your country, income, deductions and business structure. Many freelancers in the US, UK, Canada and Australia set aside 25–35% of each payment as a safe planning figure, then adjust once their accountant has calculated their real rate.
The two-account system
Have every client payment land in one account. Immediately move your set-aside percentage to a separate tax account. What's left is what you can actually spend.