NF, it depends on so many variables no one can tell you for sure how far it'll go before you lose signal. I strongly suggest you read the manual that came with your truck or, at the very least, share what kind of radio gear you have in it.
Keep in mind, battery strength, interference, weather, antenna length and the shape of your equipment all play a part. If you cut your antenna wire, you'll have major problems. If the RX is cracked, or gotten wet or excessively dirty, you'll have problems.
Chances are, all things being equal, you'll not be able to see it very well before you lose your signal. Best thing to do is keep it at a distance where, if it flips or crashes or starts acting up, you can get to it fairly quickly.
Again, sit down and read that manual cover-to-cover, man. You need to know as much about that truck as you can.