5-10 minutes max. If it takes longer and you don't hear the maintenance station working then either one or more of the other subtanks are completely full.
There is a trick to filling only one of the subtanks. Turn the printer off and restart and go into the maintenance menu (when you see the printer model appear on the screen, hold down the up, right and down arrow keys simultaneously until in the maintenance menu. Go to the "ExControl" option and hit enter. From this menu go to the "Sensor" menu. Under this you will have three options: Tanks Status, TankValveCheck and Etc. To check to make sure all the ink valves are working properly...go to Tank Status and cycle through all four valves. They should all read: "H/L/V: Off/On/Off." If one is not then you have a bad ink valve. If they are all good then go back to the Sensor Menu and hit enter. Go to the TankValveCheck and go to the 3rd valve (Magenta) and hit "enter." You should hear a faint clicking sound from below the ink cartridge well. This is the valve for magenta ink. What you are doing is turning the valve on and allowing ink to flow into the subtank. Let it sit for 5 minutes. Hit cancel and then restart the printer.
You shouldn't get the subtanklo error now. I have had the ink valves go bad on me multiple times between 5 Mutoh printers over the last 4 years and they can cause all sorts of issues.