But how long a player must play for youth teams to be considered as true La Masia player?
Technically, at least from Juvenil A. Like Puyol and Pedro.
Another question: Can a player like Vidal be considered as La Masia product (of course he is now when he returned but would he be otherwise too?) because he only played one season here when he was 12 or 13?
Yes, technically. And he's also a "product" of Madrid's cantera, but obviously that's not where the emphasis is, as he's playing at Barça.
There are also other players who have gone through both youth systems in their career [...] and even more who played for Barça/Espanyol cantera. Is a player in that case considered more as a product of the club/system where he played longest time or where he played when he was younger/older?
Obviously, the player can be considered a product of both academies, but the emphasis will be put on at which club he plays. Messi is technically both Barça and Newell's, but when he plays for Barça, the emphasis is on that he's from La Masia, but when/if he returns to Newell's, they will call him a product of their own cantera, which is also true.
What if he moves on after 2-3 seasons and never returns to Barça or Espanyol in his career but makes a solid career at other top level La Liga or foreign club(s)?
Again, both. Keita Balde, for example. Played for La Masia but moved to Cornella for a year (or two?), and now he's still generally considered a La Masia prospect, even though he also played at Cornella before moving to Lazio. I think it's because Cornella isn't relevant for him, as he's not likely to join them. They're not among elite clubs in the big leagues (most have no idea who they are), so he's just considered from Barça.
Also, you almost never find any instances where a pro player has ONLY played at one club. At Barça, I can only remember Samper who has gone from Escola to the first team. Extremely unusual. Because that's when most people even know that player exists. Those that play most of their careers at Segunda or lower level teams are usually quickly forgotten.
Can Albacete consider Iniesta also somehow as their product as he started his career there before coming to La Masia?
Yup, again, both can claim they have had an important role in his development. Albacete brought him from a smaller local club, let him flourish and put him in Barça's sight. Barça developed him in to a pro.
There are rare examples like Samper who made their first football steps in Barça and make it all the way to our first team (or at least to Barça B). Those are true La Masia graduates for me.
Then we have 0 La Masia graduates in our first team and all other big clubs also have 0. Which isn't be very sensible, I think...