Please check out this link in case you find it interesting. It's about how the moon formed from a similar impact.
In the link above, it is assumed that there was an explosive collision (moderate at celestial standards) between Earth and Theia at an oblique angle. Despite such a collision, it is thought that it took surprisingly little time to form the moon, whereas it took around 0.1 billion years for Earth to form. A corresponding collision between your two planets would likely take longer, as the creation of the Earth itself (normally) took hundreds of millions of years.
A point about Roche limits: Roche limit takes effect 2.5 radii away from the larger planet. If these planets are equal in mass, they would merge into a central mass between them. This would basically be the same as forming a brand new planet from scratch.
Edit: I forgot to give you an actual answer - sorry lol. With little-to-no actual science to back this up (we don't know much about the formation of planets) I'm going to say between 0.5-1 billion Earth years IF both planets completely shatter into debris and then coalesce to form another planet. If they merge perfectly the way you described, could take 100,000 years, as the commenters suggested. That's assuming that these two planets don't just turn into an asteroid belt or something, and that nothing else gets in the way. I'm also not accounting for bombardment of debris from the collision of these two planets, or the possibility of smaller moons forming.
I'd also like to point out that the probability of 2 celestial objects 'only' crashing into each other at 1000 km/h wouldn't be much of a collision. Is this being done deliberately? If not, incredible luck.