If you listen to my brother, about 2 years! Horses will very quickly destroy the sward on newly planted grass. The field he did most recently (about 2 -3 years ago has been mown a few times and very lightly grazed when the ground is dry, by calves mainly. He hasn't let the horses near yet. The field does look really good. If you are taalking about over seeding a fieldd which is already grass you will need to wait until the new seed is growing well and is established or it will die off and be back to square one.
As the others say at least a year more if you can allow, it needs a couple of cuts of hay/haylage/silage it needs to thicken up in the bottom to establish a good firm bed, or else the horses will just trash it.