For vs For up to // Get by in (a language) vs Can get by in (a language)
I came across this sentence in a book:

...where teenagers live in an all-English environment for up to four weeks at a time.

(Is there any difference between "for" and "for up to"?)

Can you use "can get by" and "get by"?

When I was doing an exercice to rewrite somes sentences I came across this:

I speak enough German for holidays. (get by)

First I wrote: "I get by in German on holidays", but in the Answer Key was "I can get by in German on holidays".

Can I use both?

Thank you in advance!

Question

For vs For up to // Get by in (a language) vs Can get by in (a language)
I came across this sentence in a book:

...where teenagers live in an all-English environment for up to four weeks at a time. 

(Is there any difference between "for" and "for up to"?)

Can you use "can get by" and "get by"?

When I was doing an exercice to rewrite somes sentences I came across this:

I speak enough German for holidays. (get by)

First I wrote: "I get by in German on holidays", but in the Answer Key was "I can get by in German on holidays". 

Can I use both?

Thank you in advance!

Answer

I can get by = you have the ability to get by (you might not do it for some reason)

get by = you have the ability and DO get by

For 3 years = a period of time that is 46 months

For up to 3 years = a period of time 46 months or less

Answer

"For four weeks at a time" = every time a person goes, he or she stays four weeks.
"For up to four weeks at a time" = When a person goes, he or she can leave before the four weeks is up,but can stay the full four.

I get by in German on holidays = You have already been to Germany on holiday and have gotten by with your German.
I can get by in German on holidays = Implies a holiday yet to come - you have the ability to get by if it happens.