The simple answer is the level will always go down so fill 'em up with compost enough to replenish what's been lost. It doesn't matter much insofar as when you do it, whether all at once or as you go along. If you worry about drying up and blowing away, you can always dig it in a bit. Gardening is long term and continuous, even in winter.

The only real way to know what that soil might need is with some soil tests. If your state universities Cooperative Extension Service offers soil testing that would be one step. These simple soil tests can also help,
1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drainsâ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

You want to try and maintain an organic matter level of about 6 to 8 percent and that may mean adding some compost, or other vegetative waste, to that bed every year. But how much depends on what was used by the Soil Food Web.

I put an elevated bed to rest a couple weeks ago. This one is truly elevated. About 6" deep suspended at chest height. Framed in at 4' x 12'. (it's awesomely convenient)

How often should I water it? I'm tight on the water supply. I'm assuming it needs to be kept moist. Perhaps I should just make certain it doesn't dry completely out?

I planted buckwheat and killed it off (chop and drop). Then, I covered it with hay. This picture was in spring.

Plant a cover crop. Guards against erosion, keeps weeds down and returns nutrients to the soil.

Here is a link that might be useful: cover crop fundamentals

Cover crop. Of course! Did that in my larger plot last year--growing in raised beds is still new to me, so didn't think of this obvious solution. Makes perfect sense. thanks for the link, too!

I mix grass clippings and leaves in the fall and pile it on the beds because it won't all fit in my bins. If I run out of grass I just make a big wire cage (or place tomato cages) around a bed and fill it with leaves. In spring the soil underneath is nice and fluffy and the leaves are partially decomposed. They go into spring compost batches to clear the garden for planting. You can also put down partly or fully composted stuff first, then cover with leaves. Worms will begin to till this in for you during late fall and early spring.