From what I experienced yesterday, it was totally possible in 1h30.
First puzzle was a matter of minutes, and second one wasn't hard if you divided the problem correctly.
I had a solution which didn't check graph connectivity and crossings after 1h10, graph connectivity was a matter of minutes, crossings too but I lost at least 30 minutes because of an inverted boolean. Then I had to add a strategy for last test because PHP weren't fast enough while the exact same algorithm in C++ didn't need it. So without my dumb mistake and the last test timeout, I would have finished in about 1h40 instead of 2h20 I think.
However, it depends on what you're meaning by "without any prior knowledge of it". Because though I didn't know Hashi puzzles, I knew other similar ones and that helped me a lot to find an easy strategy. Also, graph connectivity isn't easy if you're not comfortable with graphs, otherwise you can do it without thinking.