I will be travelling to Brussels tomorrow from St Pancras on the Eurostar. On the Eurostar website, it indicates that you must be at the station at least 90 minutes before hand if you are travelling on a Standard ticket:
The above screenshot are the times for St Pancras International, which gives the median time for how long you need to get to the station, check in, and get on board the train. At the other ends of the time (Paris and Brussels) the time listed to get seated is between an hour to 2 hours, depending on the station.
For those that have frequently travelled this route, is it really necessary to leave 90 minutes (or longer) in order to get on board the Eurostar?