Sage Advice gives us a guidance on how to know if a spell can be dispelled or not:
Can you use dispel magic on the creations of a spell like animate dead or affect those creations with antimagic field?
Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend.
Wish does have instantaneous duration, so it is not questionable that its effects cannot be dispelled, except one:
You grant up to ten creatures you can see immunity to a single spell or other magical effect for 8 hours.
Strangely, this is the only effect offered by wish that includes duration.
Does this mean that if I use wish to gain this effect, it can be dispelled?
Best Answer
No. The duration of Wish is instantaneous
In the quoted Sage Advice, "duration" refers to the the "Duration" field of the spell's description, not whether the spell has a lasting effect. Wish has a duration of Instantaneous; therefore, the spell is instantaneous, even though it has a lasting effect.
I realize the example you give involves an ongoing effect created by magic. When someone tries to dispel it an hour later, though, the catch is that the effect was created by magic. After it was created, the effect simply existed, without the aid of ongoing, dispellable magic.
As the Player's Handbook puts it:
Compare this to Plant Growth. It can either cause plants to become overgrown or cause soil to become enriched for one year (depending on how you cast it). The spell's duration, though, is Instantaneous. It cannot be dispelled.
Also compare to Fireball, which can burn people and set things on fire. The spell is instantaneous; it cannot be dispelled.
The bottom line
It's tempting to look at a spell like Wish or Plant Growth and say that, because the effect has a duration, the spell must have a duration. This is not what the rules say, though.
Fireball has a duration: The magic happens instantaneously. Fireball's effects also have a duration: they last either permanently or until someone reverses them (with a fire extinguisher, with healing, etc.). According to the rules as written, the addition of a timer to the effect of Wish does NOT mean that the effect continues to be magical. Wish CANNOT be affected by Dispel Magic; there's no ongoing magic to dispel.