Often I have a couple of searches running for weeks, sometimes months. And I find these cluttering up the display. Is there a way to (temporary) hide searches from the display?

No, that's not really possible. However, what do you mean with running for "month"? You realize that searches have an expiry and the maximum is 2 weeks, right?

I alway use "expires with this session", and the session usually stays for months.
If I need to restart gtk-gnutella, I restart the necessary searches manually.

Ok, but hiding active searches is rather "dangerous" because those could easily be forgotten. A search isn't free, it uses resources of the network which won't be available for others then. That's also why this expiry was added. Some comfort is necessary i.e., Gnutella isn't Google, searching doesn't work the same but there must be some limits. "Expires with this session" was more or less meant to preserve the status quo except that searches are not restarted automatically. The other choices would restart the search but then limit it in time as a trade-off. Honestly, I think it's probably mandatory to adjust the reissue interval to the expiry. In the case of a session expiry, it might be better to use a exponential back-off that wraps around because emitting a search once per hour *for months* really makes me feel uncomfortable.

Anyway, regarding your actual question, hiding is not the right thing but a different presentation of the searches might provide a better overview. For what it's worth, you can sort the searches by name, number of results and number of new results. Sorting by the 2nd or 3rd would keep the interesting ones on top.

I think you have a point. I guess it would be better to set the retry time to 24 hours, or so.

Well, 24 hours would probably be bad because you'd always catch people from the same timezone. It's also a bit extreme, every 8 hours should be fine but an uneven number (7 or 9) might be better so that it shifts over time instead of querying at the same daytimes over and over agian.