For the next 7 years I will be overwhelmingly happy with an average of 25 PPG, 6 APG, 4 RPG, 1.3 SPG, 2.2 TOPG and a TS% around 58-60%. He might have better years, he might have worse years. But if that is what he gives us for 7 years as an average, that's awesome.
So, basically, you'll be happy if Irving averages what Isaiah has averaged for the Celtics since coming to Boston:
Isaiah Thomas @ Boston
179 games, 32.2 mpg, 24.7 points, 6.0 assists, 2.7 rebounds, 1.0 steals, 2.7 tov, 59.4% TS%
Or, per-36 for his career:
441 games, 36 min, 23.0 points, 6.2 assists, 3.2 reb, 1.2 steals, 3.1 tov, 58.5% TS%
Personally, what I need to see from Irving depends somewhat on what we see from Thomas over the next few years. Because we paid for Irving with Thomas plus quite a lot more, I don't see how we can think it is a favorable trade unless we think we can get a lot more out of Irving than we could have gotten out of Thomas.
If Thomas' hip proves problematic, then maybe that won't be a very high threshold.
But if Thomas produces at anywhere close to the same level that he has over his _career_ (not even his super-elite 2016-17), then we will need more from Irving than what you are asking for for me to view the trade favorably.