I missed a dose of Effexor (venlafaxine) 75mg. When should I take the mext one?

Take when due. If you forget your dose in the morning and realize by noon you missed it, you can just take it then. If you don't realize until the evening, you can take it the next morning as you normally would. Of course, talk with your own physician about this.
See below. 75 mg is a common starting dose of effexor, (venlafaxine) and it is common to increase the dose by 75 mg/day every 4 to 7 days as necessary. People who take Effexor (venlafaxine) in the morning and realize at noon they missed a dose, simply take it right then. But if they only realize they missed a dose until the next day, they just skip it (the missed dose) and resume 75 mg once a day. Talk to the person who prescribed it.