There's what 30 codexes to produce, 10 in the first 6 months, so maybe 18-24 months to get through all of them. Atleast the armies with out a dex will be getting annual updates.
Are there actually 30? All the codexes that I can think of off the top of my head since 3rd edition (with some amalgamations), plus ones the rumour mill seems to be suggesting are on the horizon. I'm not including the small supplements (e.g. Iyanden, Raukaan)Space Marines
1- Space Marines (aka: Ultramarines)
2- Blood Angels
3- Dark Angels
4- Space Wolves
5- Grey Knights/Daemonhunters
7- Black Templars
8- Legion of the Damned
9- Adeptus CustodesNon-SM Imperium
1- Imperial Guard
4- Imperial Agents/Assassins/Inquisition
5- Sisters of Battle/Witch Hunters
7- Adeptus Mechanicus
9- Sisters of SilenceChaos
1- Chaos Space Marines
3- Thousand Sons
1- Craftworld Eldar
2- Dark Eldar
4- Ynarri (speculating that they might get their own codex at some point)Other Xenos
2- Genestealer Cults
I've got 31 there. Wow, I'm actually kind of surprised it's that many. Obviously some are relatively small and/or short-lived. And I may have skipped some.
If we assume that all of the smaller supplements will be expanded into full codices then the list expands quite a lot (especially on the Space Marine front, but also if every major Eldar craftworld gets its own book too). I can't seem them churning out that many books, but I've been surprised before.