For how much time (months, years) will Fences 3 be supported?

Until v4 comes out. You answered your own question. It's 3.05 now, it could be a year or if they decide to make it 4 next week. No one knows not even them. 

I believe you have more then enough time before it turns 4. 

But this just isn't fair. When anyone pays for this software they expect it to be supported for a certain amount of time. If we knew it's being abandoned next week, we wouldn't purchase it. A paid service shouldn't be a gamble. 

I understand where your coming from, but think of it this way if you upgrade to 3.05 and up it's yours to keep until 4 comes out. As an example the program WindowBlinds is at 10.62 and v10 didn't come out till about 2 years ago. v3 won't be abandon any time soon the next update will probably 3.05xx or 3.06 and a couple months will go by and the update will be 3.06xx. See where I'm going here? If your on Win 10 you would need a upgrade to work and for right now it's $7.49 instead of the $10 (StarDock website) and if you depend on it, it's worth it.

Btw I am not a StarDock employee I've been a StarDock user for 15 years.

Fences 3 was released about 14 months ago in response to Microsoft's release of Windows 10.

It is a safe bet that Fences 3 will be supported [current] as long as Windows 10 is essentially as it is now.

However.

Microsoft's 'idea' that Windows 10 is to be a final version of the OS means that although the name remains the same the OS itself is constantly changing.

If or when the OS is changed by Microsoft dramatically enough to cause major issues with shell enhancements [ie. Fences] then Fences may too have a dramatic revision [new version release], and that is entirely up to Microsoft as far as timing goes.

Thanks to Microsoft the definitive answer is.....not in the foreseeable future - ergo for quite some time to come...;)

Alright. I bit the bullet and upgraded. Thanks for your responses. 

Well, if you really consider it as a problem, you should consider learning programming and create your own "fences". It'll totally be much more customizable...

I made my own "Windows SideBar" far before finding out about stardock, and you know what, i see no reason to buy a software that i, as an indie game developer who never released a game, can create in a couple weeks.

Not sure if what you wrote is out of arrogance or you are simply being naive. If writing fully functional/fully featured software is so simple that you can do it under two weeks, why have you never released one of your games?

Perhaps you should - I have no doubt it would be a MAJOR learning experience for you.

You didn't have to make your own Windows Sidebar there's about 4 programs out there that are totally free. One being Windows Sidebar for 7/8/10.