Hello there,
Katha to Mandalay is a long journey and it takes about from 2 to 4 days depends highly on the level of water. If you want to make this route by boat, I recommend that you should book 2 seats because open air and seat on boat are quite narrow for foreigners. You can consider between 2 types:

• Standard seats ( it’s better to book 2 seats to be comfortable on long journey )
• Upper class cabin ( it’s narrow cabin , but comfortable , you should bring sleeping bags )

• Katha to Mandalay