To answer a question posed on the Goodman Games forums, I did some calculation about how long it would take a group of PCs to go from 1st to 10th level. If the average encounter per adventure is 2 XP, and the average adventure has 14 encounters which the PCs engage in/get XP for, then you can assume
1 adventure (28 XP vs 10 XP needed) = level 1.
2 adventures (56 XP vs 50 XP needed) = level 2.
4 adventures (112 XP vs 110 XP needed) = level 3.
7 adventures (196 XP vs 190 XP needed) = level 4.
11 adventures (308 XP vs 290 XP needed) = level 5.
15 adventures (420 XP vs 410 XP needed) = level 6.
20 adventures (560 XP vs 550 XP needed) = level 7.
26 adventures (728 vs 710 XP needed) = level 8.
32 adventures (896 vs 890 XP needed) = level 9.
39 adventures (1092 vs 1090 XP needed) = level 10.
The foregoing assumes, though, an average rate of XP per adventure, which may not be the case, and does not take setbacks such as character death or XP-draining un-dead into account. Adjusting XP given will produce faster or slower rates of advancement. Shorter adventures may not count for a full adventure using the foregoing formula. Your Mileage May Vary.
Now, to make things even more interesting, my group averages 3-4 sessions to complete an adventure. In this case, if we assume that some 40+ adventures is about right (as you want the PCs to have at least one adventure with their cool new 10th level characters!), with my group one would expect 120 to 160 sessions, if all things were equal, to reach such an exalted level. Averaging one session per week would result in a period of just over two and a half years of play.
If one instead assumes a sandbox, one cannot assume that, especially as the characters grow in power, they will be equally challenged by every encounter. In fact, some higher-level adventures may offer a paucity of XP-earning encounters in exchange for either a shorter play time or other tangible rewards (such as the world not ending).
In my experience, though, DCC campaigns (much like AD&D campaigns in a sandbox-type setting) are not about single heroes going from Point A to Point Z with no stops in-between. In actual play, I have found that most players prefer to have multiple characters active within the setting, each of which has his or her own agenda. In this way, my home DCC campaign consists of characters which have reached up to 3rd level legitimately, plus a spattering of higher-level characters initially created for playtests under the agreement that they would then be usable in the gameworld.
The result is a system robust enough to show progression, but with a progression that demonstrably slows once the characters gain a bit of oomph. In my experience, this allows game play to recall the adventures of Appendix N characters, which are often in the same power range with regards to their own worlds – powerful enough to make a difference, but not so powerful that they cannot be hurt. To me, this is very satisfying.