Most games for the PC market only run on Windows. Or Mac and Windows.

How difficult is it for programmers to adapt a Windows-run game so that it could be run under Linux?

How many man hours? And how much money will it take?

Steam made a big push to bring games to Linux (Ubuntu based..)
There are many games out there that run on linux and run very well.

It's only some AAA titles that take awhile to run on Linux. I haven't rebooted to Windows to play games in years and I play most days.

It really is just best for you to google this depending upon how deep you want to go into it.

Depends on the game.

Skullgirls and Divinity: Original Sin were both really difficult. The first for staffing reasons, and the second for things like, a) the port being done in parallel with updates to the Windows version, and b) early technical decisions (like writing the game using Direct3D) having been made to fast-track the Windows release.

Stasis is being held up by bugs in the third party engine that the game was written in.

Pillars of Eternity was an early Linux Unity title, and the team has said in interviews that it was much more difficult than they'd expected.

I have to wonder far more about how truly difficult it is to port an OSX (based on BSD) game to Linux. That said, I enjoy the often improved performance from using a Low-Latency Realtime kernel and superior TCP/IP stack that Linux provides. It's worth it to me to have reason to wait to see if new AAA titles are actually good gameplay or just glitzy graphics.

The short answer to both questions is: it depends...

The long answer is that it depends on a large number of factors. Is there an existing port to PS3/4 or MacOS? Was the game written on an existing engine that is cross-platform? If not, is the 3D pipeline based on OpenGL? How about the rest of the engine, does it use POSIX calls, libc-like calls or a lot of very Microsoft specific stuff? Does it use WinGDI or platform .Net? Because, neither have a Linux equivalent that in any way resembles the MS implementation. (Mono uses the C# language, but it doesn't implement the Microsoft specific APIs on Linux.)

Hint: Every one of those difference adds oodles of time to a port.