There's a few things in Quintana's favour, and there's a few not in his favour here.
+ He's clearly the best climber in his generation
+ He's got a very good team.
+ Reliable peaker
+ He rides pretty conservatively in races he rides, and doesn't do classics and all that crap
- limited TT, he needs to be the best climber in the race, or very very close to
- At 27, he's only won 2 GTs, and he's not the dominant stage racer (yet)
- He started his prime very early, and he's been riding 2 GTs a year
- He's prone to getting sick in GTs, and has yet to finish 2 GTs great in one year.
Then there's a lot of outside factors still up in the air. He could have a severe crash or get injured, or get form yips. Both are hopefully very unlikely. One other outside factor is going to be the competition. In the next 1-2 years, this going to depend on how mostly Froome's and to a lesser extent Contador's, Nibali's, Chaves and maybe the Yates' bro's careers develop. For the longer term, we're looking at young riders who's ceilings are largely unknown. The Dutch say 'Wat goed is, komt snel' or 'What is good, comes fast' and Quintana might see some stiff competition developing rather quickly in the next few years. Egan Bernal for example looks incredibly promising, but the development of such riders is delicate and a whole other thing in itself.
The way I see it, there's only so far you can look to the future somewhat reliably. You can look back a bit though. Quintana is entering the 5th year in which is one of the best climbers in the world, and he's won 2 GTs. Compared to other riders who were dominant GT riders, he's done a rather poor job at converting opportunities into wins. He's been piling up GT entries rather quickly, this will be the 4th consecutive year in which he starts in 2 GTs, though he's his tactics are very economical. If we're comparing to Contador, Contador entered his GT prime at 24/25, and won the next 6 GTs he rode. He did this by mostly riding one GT a year, but he's always contested every race he did hard, and very agressive. Then the world crashed down on him, he lost 2 GTs on paper, and then won 3 GTs after his dominance had ended.
Much will depend on Chris Froome and then the upcoming younger riders who will be challenging for GTs from 2019ish onward. I think that Quintana won't start thinking too much about the # of GT wins until he finally wins the Tour. If Froome keeps up for long, Quintana may never be the top GT rider in the world, and I think he'll need to be to really pile them up.
The last thing for Quintana would be how good he would be at winning the 'lesser GTs' after his peak. He won't have a TT to fall back on, but his base level is very high. After his true peak, he could probably be very competitive for a long time.
To me, Quintana's conversion rate speaks against him. But he has had no clinic gate. He's the best climber in the world, but in a sport were careers can shatter and contenders can rise in an instant, I don't think we can look ahead that far. I won't, and I have a feeling Quintana won't either.