Is possible to use the prefix multi in combination with an adjective that describes time frequency? More in detail, does "multi-daily solution" mean "a solution that spans multiple days"? Is there a better way to express the concept?
You use 'multi-' to prefix a noun, not an adjective.
So you would say a multi-day solution.
Apart from that, you are perfectly correct in suggesting that multi- can be used to cover timespans.