You can leave the light on all day, no worries, as long as you have enough water and the turtle can easily escape to cool down, it's fine. Basking areas are usually kept around 90 degrees F, and incandescent bulbs lose a lot of their heat through the sides and top so only part of it is being focused on to the basking area. You should get a UVB bulb setup within a month of getting your turtle, as longer can cause health problems.
However, have you thought through the prospect of owning a turtle? First, turtles get big and need a large tank. By time your little guy is a year old, he (or she) will need a 40 gallon tank. Full grown, 75-120 gallons, depending on size. What is he currently living in? A ten gallon tank or a "death bowl" from a street fair? These accessories you need to buy include a water heater ($20-50), a UVB bulb and fixture ($40), and a filter ($100-350). These are necessities, besides a tank and a basking area. Grand total, an intermediate setup will cost you about $500, and the ultimate setup probably closer to $1000.
I'm not trying to sway you, since you already have the little guy, I just want you to be aware that you may have seen only improper turtle care up to this point (90% of people don't care for their turtles correctly, and generally the turtles die). Please read over this site extensively, it has a lot of good details.
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