Anyone who's been in a fight will tell you that if you have your opponent on the ground, they aren't going to just sit there and wait for you to make a move. Nor is it a simple task to keep them still and preventing them from getting out from under you.
However, in movies and TV shows, it seems like at least 1/3 of gladiator fights ended with the downed foe sitting and waiting while the audience made their choice.
How often was it actually?
The gladiatorial fights that you see in movies and tv shows, are unfortunately mostly inaccurate.
Firstly, the whole thumbs up/down, comes from a single painting painted in the 18th century (or maybe it was the 19th century, I don't quite remember, but the point is that it's from a newer age). And it's this painters interpretation that has gotten way too much leverage. There are no real sources that describe how this "thumb-sign" was used. Other has suggested that the "thumb-sign" was a thumb pointed at the throat, thus meaning a sword to throat; death.
We know that a thumb-sign was used, not what or how it was.
Secondly, the fights were highly regulated. There were strict rules that applied to whom that fought whom, what equipment they used, when the fight was over, and so on and so forth. They even had referees following and judging each fight.
Thirdly, the mentality was very different from what is usually displayed. When the referee had stopped the fight, it was over. If you lost and were to be killed, you had already accepted this before the fight started. It was a part of the whole ordeal. Before the games begun the gladiators had a greeting to the emperor; "We, who are about to die, greet you emperor XXX" (freely and translated from memory).
If I recollect correctly, a documentary I recently saw said that about 80% of the gladiators left the arena alive. Though it varied depending on what type of games it was. For instance, if it was some huge important games, more gladiators probably lost their life.