How long before Ave departure can we board?

If the train starts in Madrid, it would be about 15 mins. You will go through security at the platform. Only allowed on the platform 15 mins early.

Same if it's the other way around, but you'll go thru security (much like an airport, scanning your luggage and everything) at the ground level -tracks are one level underground. This means you should arrive at the station with enough time to get thru security -which is common for all the high-speed trains- and sometimes can be a bit busy.

The procedure is:

In Atocha station, you find the upper level AVE departures. It's a large and confusing station (and I say this as someone who's used to Penn Station in New York), so allow some time for this. There are lower level AVE departures, cercanias (local) trains, etc - each leaving from a different area.

You show your ticket and go through security. They will X-ray your bags, but you don't go through a metal detector.

You then go to the departure lounge and wait for your train's track to be posted. While waiting, you can get a snack, and there's space to sit.

When your train's track is posted, you can then line up to board the train; your tickets will be checked again. Assuming you get to the line when the track is posted (i.e., you're not running late), you will have time to find your car and seat before getting on the train. This saves having to drag your luggage through the train to get to your assigned seat.

A few minutes before the schedule departure, the doors are closed on AVE trains, and no one can board, even though the train is still in the station. So, you can't jump on this train as it's departing, like you see in the old movies.

That's it; you then enjoy the ride. About three hours later (more or less, depending on how many intermediate stops your particular train makes), you get off in Barcelona Sants station. If you haven't used the cercanias code to get to Atocha, you can now use it to get from Sants to your hotel on the rodalies (the Catalan version of the cercanias - local trains that serve Barcelona and nearby cities). One line goes to Plaça Catalunya, another goes to Passeig de Gracia. There are uniformed train personnel in Sants station - just ask for help if you have questions.

To All:
Thank you very much!
Soooo helpful.