GPU mining will last as long as it will be profitable. When there will be too much miners, the difficulty will drop, and more will join. At some moment there will be an equilibrium that will rise and lower.
Too much miners will not drop difficulty level but it will rise it due to nature of crypto. Same as with btc.
Nobody could answer for how long it will be profitable but best move would be to diversify your rigs - mix of radeon and nvidia because these can be profitable at different moments with different coins.
To get the point what is more profitable to mine you can temporarily use nicehash to get that info and then get back to your usual pool.
If you are really a complete newbie on mining altcoin then nicehash would be really userfirendly since its not too technical and you can easily set-it up and talking about difficulty that its just normal to think that it would normally rise since there are already lots of miners of a particular coin and as you said we can see it also on bitcoin which means profits would really lessen up overtime but still no need to worry there are lots of coins to choose from.