How to buy a roundtrip ticket with open return date? (To Israel)

What you are looking for is called an open ticket and you'll usually have to buy it from a travel agent or directly from the airline. These are often expensive.

Alternatively, you can just book a round trip fare with the return date as far into the future as you can, making sure that changes are permitted, even if they cost. It may prove cheaper to pay the change fee than to buy a fully flexible ticket.

For example, these are the change conditions on a BA flight to Israel.

Time/date changes permitted at any time before each flight departure for a change fee of £150 or an upgrade fee of £150 plus any difference in fare. All sectors may be repriced. Changes subject to availability. Fees apply per ticket

This is very suitable if you expect to be able to know your return date at least a few months in advance. On the other hand, if you want to be able to fly back on short notice, an open ticket is the way to go.

It's probably most efficient to buy the cheapest (non-flex) round trip ticket and and just don't take the return flight. Book a one way or another cheap round trip when you want to come back.

Open tickets and flex can be quite expensive and many airlines cap the return flight at 12 months after departure, even for a flex ticket.

My personal experience was that only British Airways checked for a return ticket, which they insisted I buy, and never the Israeli Border Guards.

I suggest you purchase two one-way tickets. To Israel, at the best price you can find. And a return from Israel at an expensive, fully-refundable fare. When you get to Israel, cash it in for the refund. Then get a cheap one-way at the end of your stay.

However, you may be able to do even better. Your program sounds like it requires a visa, since the standard tourist visa is for less than one year. It is much easier to get this in advance, even if it can be obtained in Israel. And with the visa, the airline will not care about the return ticket, even British Airways.

Caveat: As part of some clever profit maximization plan, British Airways charged me about GBP 1400 for the refundable ticket, and tried to refund it for USD 1400. Numerous complaints to the bank were necessary, and a letter to BA Customer Service saying if they attempted to charge me again, they would hear from my lawyer. I suppose I should have said solicitor.