It depends where you are coming from. If you have to go through customs, I figure 60-90 minutes to deplane, get our bags, get through customs and work our way to the pickup point. At 1830, I would think (hope) the crowds were settling down.
Most car services I have used ask for your flight numbers and where you are coming from. The drivers are good at predicting how long it takes and will give you a meeting time and location.
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