Check engine light turned on for faulty transmission control solenoid

So my check engine light came on and the code that comes back is for a bad transmission control solenoid. My car shifts just fine for about the first 20 minutes while driving. When it's warmed up for awhile the shifting gets real rough and it jerks really hard when I shift. My question is, how long can I get away with driving like this? I only paid $500.00 for the car and I'm not comfortable putting a lot of money into it. I've been driving it for four months like this or about 7,000 miles.