Can we say we are not sure yet how long we want to have been spending on flowers at our wedding. instead of saying We are not sure yet how much we want to spend on flowers at our wedding.?

No

Explanation:

Because

Can we say we are not sure yet how long we want to have been spending on flowers at out weeding.

so, the word in this sentence "Have been" dosen't make sense

have a read.

but

We are not sure yet how much we want to spend on flowers at our weddings.

it would make sense, if you clearly read the sentence.

but you can also rewrite the sentence

Until now, we are not sure how long we are willing to spend on flowers during weeding.

Hope you understand

:-)

do you have options?

it might be feelings and action sorry if i am wrong