Both are rarely used in everyday language. But if I talk to somebody, I rather use "jeśli" than "jeżeli". "Jeżeli" is good for some letters, documents or official speeches.
Sometimes in everyday speech we say "jak" instead of "jeśli" or "jeżeli", but it's meaning is closer to "gdy".
Zrobię to, jak wyjdziesz z pokoju. [the least formal]
or - with the same meaning:
Zrobię to, gdy wyjdziesz z pokoju. [more formal]
Zrobię to, jeśli wyjdziesz z pokoju.
Zrobię to, jeżeli wyjdziesz z pokoju.
Everyone means the same, but two first I would translate using "when", two following using "if".
With "gdy" I point out that I will do it just when the room will be free of the other person. With "jeśli" or "jeżeli" - that the other person will have walked out of the room. In effect - the same, but sounds a bit differently.
besides. we're talking about jeśli OR jeżeli (polish; the official language of just one country)
And the EU :)