The thing(s) with grass are that it is often 'improved' cattle fattening, high sugar pasture, like rye grass etc, and that in spring & autumn especially(but also at other times, depending on weather, sunshine etc), it can be even richer than normal.
And horses in Western countries these days - like their owners - are often under exercised & overfed. Often chronically/eternally overweight, or at least 'in good condition', never getting regular 'hard seasons' to use up fat stores & 'reset' our metabolism & insulin sensitivity. Therefore, especially when on extra rich, sugary grass(or humans eating lots of carbs & sugars), we are a lot more sensitive to it & therefore we are then at risk of insulin resistance/type 2 diabetes.
**People get told to feed hay, not fresh grass, but while grasses lose nutrients when dead/processed, carbs/sugar is not one of them. The hay you feed may well be just as sweet as the grass you have, depending on the type, how it was grown, weather & when it was cut!
Nutrition does also play a part in insulin resistance, and magnesium is one of the most important nutrients for 'regulating' the metabolism(among other good effects) which is very often deficient in horse's diets.
So... I wouldn't generally worry at all about an hour here & there of rich grass, unless your horse is very 'prone'. Depends on the grass, whether she's likely to really gorge on it from having none, as to whether I'd leave her on it for long periods/full time. It's not so much about 'building up to it', but just how much she can stand. If your horse has had a 'hard season', that's a good thing for insulin sensitivity & will make it less likely for her to be so sensitive to the amount of sugars in the grass/hay, if it's rich. BUT if she is a generally 'easy keeper', she may have a bit of a built up resistance. And if she's getting grain(cereal grain is very high sugar/starch) I'd generally suggest to avoid that & feed something low carb instead.