WARNING: These calculations appear to over-estimate game XP a bit, based on my current-season guys.
All right, I built a spreadsheet to solve this. Here are my assumptions:
Daily XP progression is 100, 96, 91, 87, 82, 77, 73, 69, 65, 61, 57, 54, ...
Game XP progression is 571, 543, 515, 488, 464, 436, 414, 390, 366, 337, ...
The Daily XP is pretty solid, but the game XP varies a lot. Those numbers are the highest game XPs I saw on a brief survey of my own retired and current players (I looked at about 5 of them, including a punter).
Assuming no boosts, an odd-day player would naturally hit level 11 on day 36. An even-day player would hit level 11 on day 37.
Game XP could be higher than what I have there, but it could also be lower. Either way, the only way you're likely to hit level 14 is by waiting until halfway through the playoffs to boost. Possibly waiting for the championship game, if you don't get very close to the numbers I'm looking at above.
By the way, according to my calculations the levels should happen on
Odd-day: Day 3, 5, 9, 11, 15, 17, 21 (level 8), 25, 29, 36
Even-day: Day 4, 6, 9, 12, 15, 18, 22 (level 8), 26, 30, 37
My odd-day guys are about 78 xp behind schedule from what I calculated. My even day guys are 47-82 XP behind that schedule.