Dragon Ball Super has seen the evolution of Saiyan power that began in Dragon Ball Z jump to a whole new level. Goku and Vegeta have tapped into the power of Super Saiyan Gods, and then pushed that power even further to give Super Saiyan God it's own Super Saiyan boost (Super Saiyan Blue). During the Tournament of Power, with the entire universe on the line, Goku and Vegeta each pushed passed their limits to an insane degree, with Goku unlocking "Ultra Instinct," a supreme fight technique that even the gods of the multiverse struggle to master. Since the ToP, Goku has struggled to once again achieve Ultra Instinct, resulting in a growing fan debate.
So when is the right time for Goku to master Ultra Instinct? Let's look at the two main schools of thought on that.
On the one hand, there are a lot of Dragon Ball Super fans who think that Goku already achieved Ultra Instinct, so why beat around the bush any longer? There's a fair amount of logic to that: Dragon Ball has always been about the long, hard, road to unlocking power - but once unlocked, that power is usually supposed to be the new standard. However, Ultra Instinct is a different case, entirely. As we've already explained in detail, in many ways, Ultra Instinct could be the endpoint of Goku's evolution as a fighter. With its god-level power, Ultra Instinct is hard to surpass - but it is a perfect carrot to keep Kakarot going for the foreseeable future.
Now that Goku (and really, any other Dragon Ball warrior) knows that Ultra Instinct is achievable, it becomes the measuring stick for true max power in the series. Dragon Ball Super has long been criticized for its rapid overpowering - keeping Ultra Instinct at bay as the ultimate power fixes that. More importantly, Keeping Goku and others striving for Ultra Instinct at key times in the series, gives Dragon Ball back the kind of substance that made things like the first Goku Super Saiyan transformation such epic milestone events in Dragon Ball Z.
The more that the Dragon Ball Super anime and manga flesh out the premise and power of Ultra Instinct, the more interesting it gets. As explained by the angel Merus while training Goku, it's basically a zen state of being when a warrior can achieve a transcendent state of tranquility, during an emotionally-charged battle. In other words, Ultra Instinct is something that requires story and substance behind it, as well as the sort of introspective character reflection and monologues in major battles - a staple of Dragon Ball that fans love. If it becomes just another transformation Goku eventually masters and uses with ease, all that rich potential is lost. Not just for Goku, but for other characters like Gohan, Piccolo, Vegeta, or even Broly.
Since the manga makes it clear that even Roshi is capable of achieving a state of Ultra Instinct (even the lower "Sign" form) Dragon Ball can still retain its "wow" factor of other characters like Vegeta and Gohan eventually achieving Ultra Instinct; big group battles using the power; more applications of UI in cool battle techniques, etc. And again, since these moments would actually come only with properly-earned story behind them (and wouldn't necessarily be repeatable), they would be the kind of headlining events that Dragon Ball is built on.
Right now, Goku has master the lower Ultra Instinct Sign form in the manga. Ultra Instinct Sign (or "Omen") carries the nice risk of being wildly unstable and potentially harmful to the user, which is a nice counterbalance to the idea of Goku overusing it. Ultra Instinct is a nice enough power-up for now as far as Goku is concerned, while Vegeta is pursuing his own new Super Saiyan God forms and techniques from Planet Yardrat. We have enough power-ups for now. Keep Ultra Instinct as the ceiling that Goku chases and must put his faith in at key moments. Dragon Ball Super will be so much better for it.