How long does one have to pay the $125 per month subscription fee for Unity pro?
Technically, it is at least one year. The other plans offered are one-time payments for one or two years.
After that first year, if
- you still want to benefit from the features offered by Unity pro and/or
- your company is still making more than 100k per year and that you use Unity (for any reason)
you will need to keep on paying the $125 per month.
- you can switch back to Personal.
If I were to get Unity Pro, do I have to keep paying $125 per month for as long as I'm using Unity pro?
Now let's tweak that question:
If I were to get Unity Pro, do I have to keep paying $125 per month for as long as I'm using Unity?
No, you pay until your subscription plan ends, then, as described above, you're free to cancel unless you still use Unity (for anything) and your company has more than 100k revenue.
Or do I have to keep paying $125 per month until certain period, after which I am no longer required to pay anything?
The $125 per month payments that you are required to do are when you subscribe to the 1 year, paid monthly plan, or that your initial subscription plan ends and you're switched to a month-to-month subscription. As previously said, you're no longer required to pay anything if your subscription plan ends, and you don't work with Unity or you don't make more than 100k a year.
This is an interesting model.
There is no "I'll just pay per month and stop the subscription when I can", like other company offer (like Netflix).
One has to plan ahead what's going to happen.
If you're confident enough that you'll make more than 100k revenue for the next two years, and that you'll still be making games with Unity in one year from now, you should choose the 2 year, prepaid plan because you save money (given that you have that money upfront). And if, after that 2 year subscription, you're still confident you'll keep on doing this, you should renew the 2 year, prepaid
If your game had a big success and you made 100k a year, and you still want to push updates to your players, even if you plan that it's your last game and you quit gamedev after that, you're forced to buy the Pro license, at least for one year.
If you subscribed to a plan and you no longer make more than 100k, but you'll release a new game soon that might or might not get you back above the 100k mark, you'll have to decide whether you stop paying and revert to Unity Personal to save some money during a couple of months, and risk being required to engaged again in a one year contract, or keep on paying month-to-month and stop when you know you won't make enough money.