After your 90 day status as a tourist in Germany, how long is the time period before I can return to Germany?

Germany has an agreement with the states, you can come and go as you please with a valid US passport. If you leave and return, the period renews. It is odd that you mentioned 90-day tourist period. With a valid US passport you can stay up to 180 days or six months before you have to register with the German government for a temporary residence permit. I lived in Germany for 2.5 years. I did not file any paperwork with the goverment until after my six month period expired. I could have departed and returned to extend my stay, but I wanted to get an apartment. Oh the catch about staying in Europe. You can only stay up to 180 days if you do not initiate any rental agreement beyond the six months duration.

Just return to your home country and you can return immediately. I do it in Italy all the time

All you need is an exit stamp and then return next day for another 90 days stay.

It depends whether you need a visa for Germany or not. If you are a citizen of US, Canadian, Israel, most Latin American countries, South Korea, Japan, Australia, New Zealand or Switzerland you just have to leave after 90 days, get your exit stamp and enter the following day.
However, if you need a visa you can only stay up to 90 days per half year. That means, you have to stay out for 90 days before you can enter again.

Easy and fun-just go to a surrounding country -get stamped
and return
not all entries have places to get stamped-make sure you go to a more populous one
I haven't been stamped going from Germany to France via Strasbourg, but going to Poland via Dresden-holy cow! We had to pull off the side while they took twenty minutes examining our passports and papers (just last summer!)
The Czech Republic (via Erzegebirge) is easy,too.