There are currently no manned spacecraft capable of traveling to Mercury.
Assuming you ignored that fact, based on the time it took the Apollo spacecraft to reach the moon, and the minimum distance between Earth and Mercury, it would take approximately 804 days to travel to Mercury. A trip to Mercury could be greatly accelerated by using slingshot maneuvers around the Earth and Venus.
Other numbers which are probably irrelevant:
- Mariner 10 managed to reach Mercury 110 days after launch (flyby mission)
- MESSENGER made its first approach to Mercury 1260 days after launch