Depending on who you ask, the Roman Empire lasted from approximately 500 years to over 1000 years. This question can’t be answered easily as the definitions of ‘Roman empire’ can vary greatly.
Generally speaking Rome began in about 625 BC. Although the Romans believed their city dated back to 753 BC. But in the beginning it was only a town, hardly an empire. They controlled Italy by about 260 BC. This could be argued as being a small empire. Yet others would say it was far from it. Only by the end of its first war against Carthage did Rome actually possess any provinces (Sardinia, Corsica and Sicily) – 241 BC.
The earliest point at which everyone would agree on the Romans possessing an empire would be after their victory in the second Punic war in 201 BC. However, some claim that the empire didn’t really begin until the Augustus was declared Caesar in 27 BC.
As for the fall, the date traditionally used for this is the ousting of the last emperor, Romulus Augustus, in AD 476. However the Eastern Empire continued until about 1400 AD.
It is therefore very much left to the individual reader to decide how long the empire lasted. All you need to do is to decide for yourself when Rome had become an empire and then to work out the years.