I don't think the other 90 seconds are going to do any damage.
2 minutes is amople time to brush your teeth, especially with a motorized instrument. excess brushing can wear away the thin enamel by the gum line, causing dentin exposure and unpleasenat sensitivity. excess brushing can also cause gum recession which can expose the root structure causing more sensitivity and opening the door for other dental problems. 2 moinutes of brushing teice a day, with daily flossing, is enough.