I will tell you what I have been able to dig up but I am still trying to get better information. First the facts. Trapdoor snails (Viviparis malleatus) are freshwater, gilled, live breeders, and are sexual reproducers. Their maximum size is about 3". They prefer soft plants like algae but will eat what is available. They can live in a wide temperture range. They are one of the few types of snails that can over winter outside in temperate environments. I understand that they also do fine in higher water tempertures used for tropical fress water fish.
The next level of detail I have less specific information. You can expect them to live 10 years or more in a good aquarium environment. Ponds they have a shorter live span due to hazards such as racoons.
They do not do well in high acid, low calcium environments and in fact this can be fatal to them.
I found an interesting high school science fair project that suggest that trapdoor snails do best in low oxygen environments.http://www.usc.edu/CSSF/History/2005/Projects/J1909.pdf
Now I tend to move toward hear say and what I have observed from having trapdoor snails in my pond for a week. They prefer hard water, this is most likely to help build shell and because it is less acid so will not disolve their shells. The more they breed the shorter they live, I read in at least 2 different places but without any specifics on life span. Racoons like them. Various fresh water fish will eat young trapdoor snails but few will bother the ones 1" or larger. Carp (gold fish and koi) are among the fish that eat the young. They are supposed to reporduce only a couple of times a year.
Now for my personal story. I have 3 ponds. One is kidney shaped, it is less than 2' deep and separate from the other 2. We have a 4' high waterfall where one of the other ponds empties into the larger of the 2 and both are at least 4' deep in places. We have fish in the kidney shaped pond and the lower pond. We put 55 trapdoor snails 1" to 2" in size. We put about 40 in the lower pond and the rest in the kidney shaped one. That was about a week ago. About 3 days ago we noticed dozens of small snails the size of erasers in the upper pond and the kidney shaped pond. Today we noticed many tiny pin head sized snails in the upper pond. The largest fish are in the lower pond and that is where I see the fewest small snails and none of the tiny ones. I have a lot of guesses about what this means but at the very least I believe it means they will expand in numbers to match the food sorce they have and the suitability of the environment.