Both "find back" and "find it back" are unnatural. In other words, no native speaker would utter either of them. As @Milanya said, just use "get it back". That being said, your sentence leaves a lot of unanswered questions. For instance, how did it come about that you eventually recovered the lost purse?
1. I lost my purse on the bus, which took me the whole afternoon to get back.
2. I lost my purse on the bus, and it took me the whole afternoon to get/findit back.
The main problem with both sentences is that, as written, "which" refers to the bus!
If you want to keep the general order of words, remove "which" and replace it with "and it" in sentence #1. As you've already been told, #2 doesn't work because of "find it back".
@Milanya, please read this extract from the forum's Posting Guidelines:
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