I recon thats almost impossible to answer
I have a shuttle Zxx, they have been around like forever (about 12 years in one guise or another and parts are interchangable between most types) and they are still being produced now.
Admitidly not a particulally flash heli but great for learners.
Kalt were around for a long time and were quite popular as far as I have read but they got bought out by HPI (rc car manufacturer) and they were dumped and now it's getting very hard to get many parts.
generally the most popular models being sold will have a better chance of being supported for longer for the obvious reasons, lots of people will always need parts,
For example Raptors will most likely be supported by the manufacturer for ages as there is such a big worldwide market for spares, as we all crash at some time and you have to admit it is one of the biggest sellers by volume for a while.
EDIT: see also above answer (posted as I was typing)