How long are you supposed to stay out of Israel after your 3 month tourist visa has expired in order to get another tourist visa?
I'm a U.S. citizen, if it matters.
US citizens don't need a visa to Israel, and get admitted for up to 3 months (90 days) at a time. Once you left Israel - you can be readmitted for additional 90 days, but leaving through land crossings for several days in Jordan/Egypt might not count as "leaving" (similarly, by the way, as the US treats foreign tourists leaving to Canada/Mexico and then coming back).
If you're leaving by air and coming back by air then you should be fine, but if the border control suspects that you work/live in Israel (i.e.: you keep leaving for a week or two and then coming back for another 90 days) they might deport you. Once deported - you'll probably have to obtain a visa prior to boarding before coming back next time.
If you're in Israel - you can extend your stay at the population registration offices (they might deny your request, so keep that in mind, especially if they suspect you to be working).
Based on the Israel's MFA website
A B/2 visa is valid for up to three months from the date of issue. The duration of the stay in Israel will be determined by the Border Police. A visitor who wishes to extend his visit may submit an application at one of the regional population administration offices of the Ministry of the Interior.
So just follow the instructions listed on the page and you should be good.