Most likely a drop of 5 points or more in training camp is a bad sign, but it's not impossible that they would recover that. The bigger the drop (or gain) the more likely they will trend in that direction.
Most players should stay within 10 points or so of their original future, but occasionally you will see a player that has his future growing faster than his current and could have a huge gain. I don't know if I've seen as high as 50, because most players with less than 50 future don't get a chance, but I have seen 20+ on some late round picks.