How long did the Roman Empire go on for, and what caused it to collapse?

De facto (in fact) from (70BC- 454AD) so...524 years. 

The Roman Republic of course was before the Empire and went on from about 400BC- 70BC...470 years. 

I say de facto (in fact) because historians are forever writing this or that date as 'official' just because someone wrote it down that way, who didn't know it later would be called official and it depends on which document you read etc. So, it's better to stick to well known facts on the ground so 524 years (70BC-454AD) for the Empire.

Why did it collapse....failure of reform several times over several 100's of years from the 'Fall of the Gracchi' (121BC), to the Servile War of Spartacus (71BC), to the 'Disorders of the 200's AD....the elite ruling rich of Rome just would never share their wealth, allow the public to own land, allow real voting/democracy, had way to many slaves, and couldn't control their professional army except by bribes/ corruption.

Empire and Rome didn't really ever 'collapse' it just gradually fizzled out because not enough wanted to fight for it or keep a rotten system going.

It split. Do you mean the Western or the Eastern Roman Empire?