Talon is a strong laner and a strong roamer but some matches are very difficult. You wont learn if you dont just play him. You might start off with poor scores and mostly losses but you'll learn and get the hang of how to deal with certain matchups eventually. Generally if enemy has some tanks you will want at least some AP on your team before you pick Talon but that shouldn't stop you from playing him anyway. Just gotta end sooner but you gotta end ASAP as Talon anyway since he falls off the later the game goes.
But some champs you will just want to ban as Talon..like LeBlanc and Fizz. Hate those two.
But in short...there is no 'when should I pick him.' Just pick him. Even better if enemy is very squishy.