If I study for half a year, reach 1800, then don't play for a year, will I forget it all? How often should one play to maintain playing strength.

I am perhaps the worlds foremost expert in this. I kid you not!

I am 50, I learned chess at about 7, and my pattern has been to play for 6 months, then quit for 5-10 years at a time. I quickly reached OTB 1600, and there I have stayed for 40 years in this pattern.

How long to regain strength? If I leave chess for 5 years (And by that I mean not even a single game of chess in those years) I can regain my former playing strength in about 2 months, maybe 3 if you are including having some decent opening lines memorized.

That leaves me with 3 months to play in some tournaments, before I abandon it again for another 5 or 10 years.

Tournament years for me were - 1978,1981,1987,1991,2002,2007, then 2009 and now 2012 (as you can see, the last 5 years has been a burst of consistency unlike any I have ever achieved!)

Now be prepared, when you leave for 5 years, then suddenly play a game, you will be a total noob. It does take some time to get board vision up, like I said for me two months to regain all my strength. If I enter a tournament before then I would expect to embarass myself.

Openings will dissapear past about 5 moves, tactical vision will be horrid.

Tactics is easy to replace, just keep CT-ART or something on file. Daily practice and it will all come back to you.

My theory is every player has a natural strength level, based on what? talent, whatever you want to call it. Getting back to that level, once you have reached it, is pretty easy. Pushing past it...well that is more problematic.

Sounds quite reasonable, InvisibleDuck. I do want to improve my chess but I have a tendency to drop things for extended periods, so I don't want it to be all for nothing :p
But I think I can commit to doing a few tactics problems a week so hopefully I can maintain my strength that way. 

When I was a begginer level player I stopped playing for about a year, and when I started playing again I was better than ever. Go figure.

I wrote a detailed reply, but it got eaten. I'll summarize. My rating did not fall at all in over ten years of inactivity. But I was rated about 1250, which is probably well below my natural talent.

I think you hang on to principles forever, but you lose your familiarity which lets you spot stuff fast. You keep your knowledge in your hard drive, but you lose it from your RAM. You need practice to put it back into RAM. Your brain won't keep it there if you don't need it.