I think the definition of "backpacking" for most folks here is hiking and camping in the wilderness. Is that your plan or are you simply touring Europe with a backpack via public transportation?

If the latter is true there are probably other sources that can provide you with better information than a lightweight backpacking board.

How much time is "enough" is a pretty subjective question. I think only you can answer that.
_________________________
If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.