To be sure, ask the contractor if they will take care of wallpaper removal. Even if they do offer wallpaper removal before plastering, it isn’t too difficult to do yourself and will save time and money.
Weather can affect how fast plaster dries. Any humidity or precipitation is going to increase the time it takes plaster to dry. To make plaster dry faster, small heaters and good ventilation can be used to encourage drying.
To become a certified plasterer, the tradie would have been under apprenticeship for several years and earned a Certificate III in Solid Plastering. The total time depends on the employer and exact type of certification they want to achieve.