I am relatively new sorry for the question.
When I paid for energy how long it is effective, a year or two or definite?
It is renewable, think like buying LEGO, it is now yours to keep until you lose a piece 
So as you use the widgets, the energy is depleted but as you recycle the widgets, the energy is returned.
This is why purchasing more energy is not like other DLC type processes or monthly subscriptions… But more like allowing yourself a larger collection of “keeper” projects that you don’t want/need to recycle as you learn Blynk.
NOTE, there are a few cases that the energy used is NOT renewable… but you will always be warned prior to committing.
There is also the possibility of long unused accounts getting disabled over time… but generally that takes many months of total neglect to even login.
Good news. Got it thank you.
Can I use the energy I bought for other projects?
The energy is attached your account, so any projects you create in that account will all share from the same energy pool.