Using Mark Lynas' book Six Degrees and the representative concentration pathways from the IPCC report long term for me is a century. The earliest we would see 5 degrees of warming would be around 2100 under RCP 8.5, the business-as-usual pathway. At that amount of warming we can expect a Cretaceous-like climate where methane eruptions destroy infrastructure in the northern hemisphere, super hurricanes flood the coasts, and hydrogen sulfide from the dead oceans suffocate terrestrial life. Of course, we likely are no longer on the business-as-usual pathway, we are taking some limited steps to mitigate climate change so I would say more likely long-term means 100-500 years, unless we significantly make changes to put us closer to the RCP 6.0 or RCP 4.5 pathways which are merely disastrous instead of fatal.