is this interchangeable with når som helst?
In this case, yes, but not always. "Når som helst" can also mean "any time now":
If I were to use når som helst would I have to say vi kan spise middag TIL når som helst or do I not need the til?
"Vi kan spise middag når som helst" = "We can eat dinner any time (we want)", no need for 'til'.
I think "når som helst" is more informal and thus used more often than "til enver tid".
Kan jeg si "Vi kan spise middag hver tid"?
I think it means more We can eat dinner every time. Like you can even eat dinner at morning time.
An English phrase is at any old time which maybe should also be accepted