If you're looking for a meeting place or a good stop around ½ or halfway between Mindelo and Cidade Velha, you can try Santa Maria, Cape Verde, which is about 9 hours east of Mindelo, or about 10 hours north of Cidade Velha.
Another possible place to stop near the midpoint is Espargos.
If you drive straight through without stopping at all, the total distance is 464 km. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 18 hours, 58 minutes.