It can take between couple of minutes up to 24 hours. It depends on the work-load, whoever is online right now, and on the stuff that is in your modpack. (If you have third-party mods that aren't on the list it takes longer)
Mine that I all mark as beta even though I check over them but like to state beta encase they need adjustments. They all take either a few hours to at least 12-24hours. Otherwise I just missed a few details for the descriptions but other than that they have all been uploaded so far for the ones I have. 1.7.10, 1.10.2, 1.12.2 the version and the other requirements all check out and when updating one in particular the most it seemed like just a check of the important details and the 'include' per update of mods added really so it was quicker since it was an existing modpack but updated. That's my experience and I've done it with about 5 or so pack so far.
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Niche Mod Finder/Acknowledger/Tester/Player. Fan of Datapacks, Rift 1.13/1.13.2 Mods, Fabric 1.14 Mods and interesting underrated or new 1.12.2 and below versions of mods. :) Testing 1.13.2 Forge and Fabric 1.14 mods mostly though.
I sometimes create the odd modpack or adventure map once an idea comes together.