Supply change how long

My first bill came out on the 28th. How long does it take for the utility supply to be changed over

If your bill came out on the 28th you changed over on that date.
Suggest you send meter readings on the 26th of each month to keep your bills accurate, rather than estimated bills

That all sounds good but the smart meter still showing as it’s the old supplier

Have you got smets 1 or smets2 meters? If you have smets1 the IHD will show the old supplier
If you have g1 type bulb will be unable to read these and you will have to submit manual readings