Note that the situation in Cuba could change, given recent opening up to the US and with the recent passing of Castro, and as a result you should probably check with your government, or a Cuban embassy, but for now, these are the rules, as according to Wikivoyage:
A tourist visa card (visa de tarjeta del turista) is necessary for travelers from most nations. This visa, which is really little more than a piece of paper on which you list your personal information, costs between 15-25 CUC (or €15-25), depending on where purchased. It can be purchased at the Airport in Cuba on arrival, however it should be noted that many airlines will require a valid tourist visa card before boarding flights. It is usually valid for 30 days and can be extended once for another 30 days at any immigration office in Cuba (for 25 CUC) - beyond this you would need a flight out of Cuba within the extended visa period. Canadians are the exception, getting 90 days on arrival and can apply for a 90 day extension. Your passport needs to be valid at least six months past the end of your planned return. Canadian passports must be valid for at least one month beyond the date of expected departure (1).
From Canada, the tourist card is normally provided on the flight. It
can also be purchased from most Latin American gateway airports if
departing from there (Cancun: 250 MXN, Mexico City: USD 25). Please
note that if departing from Europe (this may apply to other
countries), you will require to have the visa before boarding the
plane. Some times, the airline provides these at the airport, however
check first that this is the case. Without a valid visa, boarding will
be denied (the airline would otherwise get a $1,000 fine from the
Cuban immigration authorities).
In my own experience, I flew from Tijuana, to Mexico City. In my layover, I had to find a booth for tourist cards to Cuba, pay, and get a receipt + proof, before my onward flight to Cuba. It was very straight-forward.
Germany: You can obtain the tourist card through the Cuban embassy in
post. Travel agencies may often offer cheaper and quicker services
and regarding continuing to extend:
Regular tourists who renew their 30 day visa are eligible to depart
the country (to any destination) and return immediately enjoying a
further 60 days (30 days plus a 30 day extension). You are only
allowed two consecutive stays in this manner.
As noted above, "beyond this you would need a flight out of Cuba within the extended visa period". So if you extend, they'll need proof you're leaving.
Of course, you're hoping to return and do it again. At this point, like any country, it's likely to be up to the discretion of the immigration official. They'll want evidence you're not trying to 'live' in Cuba on a tourist visa (the US for example is very strict about checking this), and so evidence like a job at home, your rental/house evidence, further evidence about plans to return home in a timely manner may all help your case for access, and while there's no law against it, and no specific 'waiting' period between visas - you'll need to convince them you have a valid reason and aren't just trying to stay unofficially.