LÖVE

anyone?

Depends. It could be an extremely polished version of pong with different modes and other bells and whistles, or something more ambitious, but might come up short in terms of content. It's entirely gonna depend on your, or your team's skill level, and how much time you can commit to it on a daily or weekly basis.

There are some games that have been produced in a really short time frame (Mega Man 7 was developed in three months), but usually such projects are either small in scope, have big teams working on them, are pressured by strict release schedules (most licensed games), or have a well-established plan or vision to build towards.

I think my usual advice to questions like this is to pick something you can reasonably do a quick prototype for in a week or two (just the basic gameplay loop to see if it's fun to play), and then just polish and add content to that foundation.

A solid 8 to 12 months of fulltime, or weekends and evenings?

Also some factors to consider:
Are you working alone, or with a team?
Do you already have assets available, or do you make from scratch?
Etc.

It's totally depends on your team size, with how much time you are spending in a day, you have some codes as readymade, how much professional in your professions.