As far as the timing goes, now was probably one of the better times to jump in. First, by not jumping in immediately he didn't come across as, shall we say, "desperate" to get the nomination, and desperation is always a turn-off, be it in dating or in politics. Second, by waiting he allowed the field to narrow itself without him having to participate in the winnowing; he comes in more rested and with fewer minor dings on his political reputation. Third, he hasn't become just another face in the crowd, just one more name to be rattled off and passed over as one lists all of the aspiring nominees; overexposure can make it difficult to get the public to focus on what you're saying. Finally, if he'd waited much longer it would have been too late; Romney was slowly but surely becoming the inevitable nominee solely by default, and if Perry had waited much longer, Romney would have cemented that impression in most peoples' minds and Perry would have a much harder time - and a lot less time available - to overcome that appearance of inevitability. If he had waited until Romney managed to get that appearance of inevitability, the big-money donors would have started to pony up for Romney because it would have looked like he was the only viable game in town; once that happens it would most likely be very difficult to get the big-money donors to drop Romney because that would require them to admit that they had been wrong with their first choice.
No, I think Perry picked a very good time to jump in (maybe not the perfect time, but certainly better than anyone else's timing) and I don't think his ratings will end up tanking. Leveling off a bit, yes, as he gets more and more vetted some of the sparkle and shine will wear off; however, the real substance underneath that sparkle and shine could very well appeal to other voters who were initially turned off by the sparkle and shine, so the net effect on his numbers most likely wouldn't be that negative, and could very well be positive.
