Dirt jumps with wood

Properly treated timber will last decades in the ground. Wood treatments are generally rated for there preservation ability. Different treatments are used for protection from insect and moisture damage, and the timber is treated to suit the local conditions. Most countries have a set of standards - e.g. American Wood Protection Association for USA. Some timber will last several years in ground with no preservative, and a lower level preservative treatment might last long enough for your purpose

As far as longevity - the problem really comes down to safety - who will know when the timber has degraded, who will check, and what damage will result form a collapse?. Also you need to consider the construction methods - building something to last a few days is very different to building something to last a few years - do you have the wood working skills to be sure you will built the structure strong enough to last years of jumps.

While you could build a structure in timber, cost factors into it. You not only need timber, you need the fasteners and time and tools to build the structure. Dirt is cheap, and for jumps versatile and easy to change profile - most likely it will be much cheaper to get a truck to drop off the dirt you need to build the jumps, but do cost out having the dirt taken way when you no longer want the jumps. It your that way inclined, its also a decent an excuse to hire an excavator and have some fun...

(mattnz already answered most things but this is too long for a comment so here it goes)

Has anyone seen this/have any feedback?

I built this once just to see what would happen. Also have used wood for filling up (as a beginner). Observations:

• it works, but not really worth the time or effort. Unless you're really building a huge table probably.
• all in all, if you spend the time making a wooden construction already there's really not much benefit of stacking dirt on top of it, apart from it looking like an actual dirt jump and the possibility to slighly alter shape: could just as well finish it with wooden plating to get wooden jumps and landings. Extra dirt on top at that point is just more work, more repair, harder to do.
• unless you use properly treated (hard)wood which also is rather expensive compared to dirt, it's not going to be much fun. Using untreated wood as a filler is a big no: once it starts degrading you need constant repairs. Or worse, it might break or break out under load (especially when not enough dirt covers it) and when you and your bike are that load there's a myriad of ways to crash :)

Is 18-24 inches enough dirt to properly compact and hold?

Assuming you mean this is the amount of dirt after compacting: should be about right. More would be better but requires even heavier construction underneath. And even treated wood tends to work a bit leading to cracks. Also I have the impression it tends to dry out faster than one big pile of dirt. Another factor (though given your climate that might matter less): I like dirts which are integrated in woods for example and covered with plants on the side. Also helps with erosion from rain. That's less likely going to happen with a wooden structure underneath because there's not enough moisture/nutrition for the plants.

Since you mostly talk about a bridge: don't underestimate the kind of construction that needs to be really stable. If it's just to guard you on those first attempts where you might not clear the gap it's simple, but if you're actually going to use it as a tabletop and do tricks on it you should know your way around with building outdoor wood constructions which last under load or it will start wiggling in no time.

One last thing since we're talking wood: I did once do the opposite i.e. use wood on top of dirt. We wanted to have a quarter pipe usable for tricks one the deck and coping, and that's impossible to build in dirt. The shape just gets lost in no time. Putting a wooden deck on top works out nicely then.