In short, there is very little difference, if any, and I wouldn't worry about it. Only as a noun ( e.g. until forever) should it always be written as forever.
The Oxford English Dictionary says:
5b: in phrases, for ever (sometimes, esp. in U.S., written forever, q.v.)
The phrase for ever (see ever 5b), written as one word. Chiefly U.S. exc. in sense ‘incessantly’.
quasi-n. Eternity, perpetuity.
I think most readers wouldn't even notice whether you used one spelling or the other. But the OED indicates some subtle, potential differences: it is spelled for ever everywhere and in all senses, but the spelling forever for the adverb is more common in America, whereas in England forever is more likely to mean "incessantly", so in a sentence that describes a repeated action. As a noun, it is always forever.