For example, I say "If I'm ever successful.." and a response of encouragement would suggest that the only question is when I will be successful, rather than if I will be successful.
When, not if - Relates to a temporal vs. a conditional case. You're trying to say "there's no way this can go wrong - It's just a matter of time until I'm successful."
The best way to literally translate this would be:
"Wann". Nicht "ob".
or even as a nice alliteration
"Wann". Nicht "wenn".
A less literal, but more common saying would be:
Es ist nur eine Frage der Zeit, bis ...
German has three main relevant conjunctions.
"Falls" is an explicitly conditional 'if'.
"Sobald" is an explicitly temporal 'when'.
"Wenn" can mean either and is about equally likely in either role.
Both "falls" and "sobald" are much less common than the all-rounder "wenn".
The problem with translating this phrasing is therefore that you'd probably have to replace "wenn" in one role with "wenn" in another role, which doesn't work to give the motivational aspect you want. Only if someone happened to have used "falls" you could reply with "sobald" to express what the English saying expresses. Usually, a German conversation must find other ways to convey this particular point.
I'd say "wenn, nicht falls" to express that it is out of question you'll some day be successful.
In general I'd follow this rule: If you can replace if by whether, use the German ob. If you can not, use falls or wenn. When can be translated as wenn if used in a conditional context or wann if used in a temporal context.
I wonder if (whether) he will be on time.
Ich frage mich, ob er pünktlich sein wird.
I'd be happy if you could help me.
Ich wäre froh, wenn/falls Du mir helfen könntest.
I wonder when he will arrive.
Ich frage mich, wann er ankommt.
I'll pick you up when you arrive.
Ich hole Dich ab, wenn Du ankommst.