We were staying at a Homestay near Ummed Chowk. The host was more than happy to show us the way towards Mehrangarh Fort so we set out walking with him in the morning.
It is one of the largest forts in India. Time required for exploring the entire fort would be minimum 4 hours. But depending on your liking for such places, you could even spend an entire day here.
The Taj Mahal of Marwar, Jaswant Thada is at a walking distance from this fort so could walk up to it. And then end your day watching the sunset from the nearby hill.