If you mean FBA, only amazon can decide that based on where the stock is, where the buyer is and whether or not they have prime
It also requires your stock to be standard fba and not s&l although I do have a few s&l enrolled asins that are on next day

You need to stop worrying about the prime delivery speed as you have no control over it. Yes you can change from small and light to standard FBA but you still not guaranteed next day delivery

The best benefit of Prime is increased visibility and a higher level of sales that you get with FBA.

Not every customer wants an item next day.

Prime doesn’t always mean next day. You need to change your mindset when you think about prime delivery.

99% of customers want a next day deivery and you can see it selling it. If the delivery is later, the sale go down.

All you can do is give enough stock that Amazon can spread it around their network so there is always a unit close enough to a buyer. Other than that, there isn’t a setting somewhere - Amazon will deliver it as fast as they can.

as everybody else has stated ,
the vast majority of Amazon FBA is delivered next day apart from small and light . which often has a longer delivery promise , this i believe is because they use a lesser postal service like Royal mail 24 which is not guaranteed.

I agree some people are willing to wait, however a lot of people want items quickly.

As others have mentioned there’s is not a lot you can do when controlling FBA delivery promise dates. You can send a larger batch of units on an in bound shipment, Amazon will then divide your inbound shipment to different fulfillment centers and you will cover more of the country with quicker delivery times.

Next Day delivery and Free delivery keeps getting pushed by Amazon saying its what customers want.

However, go to many of the big stores sites and you often find to get free delivery there is a minimum spend. To get next day delivery there is a premium upgrade option. Strange that many stores are doing the opposite of what Amazon says is true.

Free deliver is a con anyway - sellers just add on the postage cost to their prices. It really should be described as Inclusive delivery and not free delivery in my opinion.

But going back to Prime - yes I can see how with certain goods it may give an uplift in sales. But it is by no means a guarantee.

I used to do Prime - but when Amazon brought in that Prime orders had to be sent at a weekend, I removed Prime and reduced my prices by the extra amount I had to add to send tracked. I now send Parcel48 as standard with option to upgrade to one day delivery. Its made hardly any difference to number of sales. 80% customers accept 2 day delivery. 20% pay for the upgrade to next day delivery.
So my advice is dont assume it will make a difference to sales based on Amazon advice - trial both methods and see for yourself.