Fresh plaster needs time to dry completely before you paint, otherwise trapped moisture can cause peeling, cracking or patchy colour later. For small repairs, this can take a few days; for larger areas, it often takes at least 7–10 days, sometimes longer depending on thickness, temperature and humidity.
You’ll know plaster is dry when the colour changes from dark to a consistent lighter shade and it doesn’t feel cool or damp to the touch. Rushing this step is tempting when you just want the wall finished, but it usually leads to frustration later.
Once it’s dry, apply a suitable primer or sealer over the repaired patch before your main paint. This helps even out absorption so that the new paint doesn’t flash or look different where the repair was done. A little patience here leads to a much more seamless result.