After the experience, it may be better to state the rules, costs, and boundaries clear from the beginning, in relation to extended assessment over time. I figure you did not do this on this occasion beforehand, and thought on helping them out post meeting as a friendly thing to do, but if it conflicts with your personal time, or the amount of time you can freely provide, then you should probably start setting those boundaries now, and lead them on to either self-learning resources (books, web sites, forums such as this one) or private tuition and/or extended sessions if you consider. Simply stating that you have limited time, and that to attend them properly you have to set things aside that you need to make up for, seems like a comprehensible argument.