Not really repopulation but couldn't think of better word. Thinking more along line of post-drought redistribution (migration?) throughout the lake.

How long does it take fish to "repopulate" a badly drought depleted lake? Say a lake down to 30% capacity, then rains like we've been having bring it back up to 100% (or more) within a week or two. Do the fish spread out as the lake refills or, for a while, hold to where they are as when the lake was at 30%?

I think the fish will elevate in the water column and spread out. Bait fish will be key with fast rising water.

The fish will flock to the newly flooded vegetation in search of food.

Lots of variables. If the the drought pool went hypoxic you may be left with little to no game fish to act as brood stock. Then you're be left with rough fish (gar especially) who handle poor water quality well.
If somehow gamefish and baitfish were to survive you would experience what is often called the new lake syndrome. Flooded vegitation and woody structure act in a way to culture a boom of baitfish, and gamefish. In this scenario fish will seek shallow structures (except palagic sp like hybrid striped bass etc) as it will harbor greater food sources. Seasonally this will continue as water permits.
Your restocking times will vary due to brood stock numbers, bio load, water quality, topography of water body etc.