Proposed system: 100 xp/month and level: you level every 10 months of real time adventuring, no matter what. My current 1st level character (who´s been played for 1 month) will level in November 2005, then again in September 2006, then again July 2007. He would be 5th level in June 2008, 10th level in September 2012, 15th level in October 2016, and 20th in November 2020.
Proposed system: 50 xp/level and month: This just takes double time to level: 20 months per level, no matter what level. My current character will level in September 2006; reach 5th level in October 2011, 10th level in February 2019, 15th level in June 2027, and 20th level in October 2035.