It depends on the item size, weight and the seller etc, but to be safe, it is best to generally allow 3-4 weeks for shipping.
I wrote an article here about how to deal with the long shipping times:
How to deal with the long delivery times when dropshipping; case studies
About the returns, you need to treat it like you would any other business, you are going to need to accept return at some point, if you are in Malaysia and the customer is in Malaysia, there are a few options; you can have the customer send the product back to you for a refund, you have to issue the refund, but at least you have the product. If the product is damaged or the supplier sent the wrong product, the supplier will usually issue a replacement or a refund. In that case, yes you have to issue a refund to the customer, but at least you also get a refund from the supplier. The aim of the game is to ensure that your profit margins are high enough that the returns don’t matter at all.
I will write an article about it at a later date, but I hope this information helps for now.
– David