I believe in Berlin it's 6 Months out of the year or something (with caveats attached).

There is no time, but a space limit in Berlin. You can rent out as much as you like if it's less than 50% of the space of your apartment you're actually living.

If it's over 50% or you are not living there at all, you need a permit.

The 90 day limit in Greece is related to how income from the properties is taxed iirc.

Not exactly:

http://www.haniotika-nea.gr/90-meres-to-chrono-i-enikiasi-me...

It's 90 days, 60 days on small islands and you can only go over that limit if you're low-income. Compliance is a different issue, because Greece, but hopefully AirBnB and the other platforms will help police this.