Generally, as long as you want. Some counties (UK for example) allow citizenship to be revoked by the government unilaterally (i.e. they think you're a terrorist, and you have citizenship in Another Country. Cannot lose U.S. citizenship unless you revoke it, in person, outside the U.S. at an embassy.
If you hold German citizenship, whether dual or not, then there are no occupancy requirements.
You can apply for dual citizenship to the USA and almost any other country. Many more people than you might think hold dual citizenship. But the road to dual citizenship, with only a few exceptions, is long, twisting and full of "gotchas".
The US recognizes dual citizenship.
Yes, as long as you are not a defector.
Since you have British citizenship, for an unlimited period of time.
India does not allow dual citizenship.
Yes, Thailand allows dual citizenship.
Yes, Venezuela allows dual citizenship.
Yes, Poland allows dual citizenship.
Dual citizenship is when one has citizenship rights in two countries.
Yes, it is possible to have two passports for dual citizenship.
Being already a Filipino citizen because of Dual Citizenship, You can stay as long as you like.