There's no magic number here. It depends on what you are selling, where you are selling it, how it is priced and market supply and demand.
Broadly speaking, if you price your campervan above market, then you can expect to wait longer - anywhere between 2-5 months. If it's at market value, probably between 2-6 weeks. If you price it under market value, then it will probably sell very fast. A quality campervan at the right price can sell within 24-48 hours!
Do take these numbers with a large pinch of salt - the takeaway here is that your pricing strategy impacts the time it takes to sell your campervan - by a lot. This is no different to selling other types of products - good deals go fast! What strategy you choose is entirely up to you and will vary from person to person.
For more information on this topic, we recommend viewing our seller's guide.