I personally have an 8-10 hour gap between applications of most TDs, but that’s only because of convenience. I use several Apex products (Helios, Resveros, Ursa, and currently using Arimahex as well) at once so I space doses out as I’m already covered in TD deliciousness. Ideally, you’d want to wait several hours before showering, but I haven’t noticed anything that would suggest things haven’t mostly absorbed after a couple hours. That’s just going off feels though.