You are landing at 6:15pm...by the time you clear customs and immigration, it will be about 6:45pm. If you get a Grab (I don't use Bolt) or taxi, it will likely take you 20-25 minutes to get to where most of the terrible traffic can be (you are doing a reverse commute/drive here; most traffic will be coming OUT of the city, not going IN at this time on a Wednesday evening). You'll likely sit in 20-25 minutes of traffic, but I cannot imagine a car ride taking more than an hour, max, for this journey. If you'd rather take public transport, take the ARL to Phaya Thai for less than 50 THB and either grab a taxi down from there or jump on the BTS to Phloen Chit for less than 40 THB. The shuttle bus to/from The Conrad picks up from there (in front of the big Siam Pharmacy) every 15 minutes. Just tell the driver you are checking in and I suspect he'll let you on.