BTW its more than just "Iran vs Arab". It's pretty much world and regional powers. After what happened to Libya and other Russian allies, the might and Influence of Russia has gone down drastically. By keeping Assad, Russia will prove its might.
The Iranian military industry is quite active as well, so they wont run out of options for weapons. With the Russian navy protecting the syrian ports, they could even send in armed vehicles.
Its bassicaly (shia vs sunni, Turkey and allies vs Iran, USA vs Russia...) the Syrians are fuel for it.
Now lets look at similar scenarios that occured in the near passed.
Algerian Civil War
Date 26 December 1991 – 8 February 2002 (continued low level insurgency)
(10 years, 1 month) took over 100,000 lives and ended up with government victory against "islamic fighters"
Iraqi Civil war.
2004- 2008 about 4 years with low-medium level insurgent made up of terrorist attacks against civilian areas.
At least 250,000 dead
Lebanese civil war (most realistic and similar scenario for Syria)
Date 13 April 1975 – 13 October 1990
(15 years and 6 months)
about 120,000 (that is an extremely large number in proportion to the population)