I started taking lexapro (escitalopram) 6 days ago and just started to feel nauseous today. is it normal to have this delay in the onset of side effects?

It is possible: It is possible. Different Lexapro (escitalopram) (i.e., serotonin) receptors have different affinity to the medication; not all of them are saturated at the same time. Also, the drug concentration may go up for many reasons (did you have your grape juice in the morning?).

Not abnormal: Feeling of nausea after taking escitalopram are not uncommon and can absolutely occur after 6 days. Try taking the medication with food or water. If you continue to have feelings of nausea speak with your doctor.