This obviously depends on the complexity of what you want to do.
If you start directly on marketplaces, there is no time limit . From the moment your account is opened, and you have passed the "Verification / KYC" step, you can start selling on most marketplaces like Amazon, Cdiscount, Fnac, Rakuten, eBay ... For some marketplaces, you will need an interface via an integrator: La Redoute, Rueducommerce, Spartoo ...
If you want to interface your website and manage via flows, the easiest way is to go through an integrator. It will take an integration time, aiming to adapt your flow to what is necessary to sell on the marketplaces. The deadlines for this stage range from 1 day to several weeks, sometimes several months depending on the complexity of what you want to do, the quality of your catalog, your ability to export the mandatory attributes required by marketplaces.
The time to set up the sale on marketplaces depends on several factors:
Depending on the marketplace chosen, the reactivity time can vary greatly. The complexity of your situation can be a source of time to anticipate.
If you have a website with a product feed to distribute on marketplaces, you must already know whether the item sheets are already created on the marketplaces, or whether they will have to be created.
To check if your products are already referenced, just look for the EAN code of your products on the marketplaces. If you find them, it means that the article file is already referenced. Otherwise, it will have to be created.
If your products are already listed, it's easier, and in a few days your feed can be configured to be sent to marketplaces. The sale of your products on marketplaces can then go very quickly.