Polux is said (on Lexicanum and 40k wiki admittedly) to have served for eight centuries as Chapter Master of the Crimson Fists. Although Dante is probably older at 1300+ years, I don't know who has served in the post longer. How long has Dante been Chapter Master?
- Polux (800 years) < Dante (1100 years)
I assume you are referring to Dante of the Blood Angels. If that is the case this answer is according to Lexicanum on their respective profile pages.
Polux - Polux died eight centuries after the foundation of the Crimson Fists, during a battle for an unnamed system, codified as HR8518. (Taken from the second to last paragraph just before the miniature picture, No source is present)
Dante - Commander Dante is one of the most experienced and able Space Marine commanders. In no small part, this is due to the longevity of the Blood Angels, which he has ruled for 1,100 years. (Taken from the first paragraph, Sourced from Blood Angels 3rd Codex)
Though there is also a quote on his page that says "For eleven hundred years, I have fought..." This would tell me he has been a battle-brother for that time and less time as the Chapter Master. Dante was made the Chapter Master after the Kallius Insurrection but that I can only find happened in M40. However, Dante is still alive and kicking, and we are officially over 100 years into M42 now without word he has fallen. I give him the benefit of the doubt...