How far ahead do I need to book trains and buses?

You might miss out on the very cheapest advance fares, but there will almost certainly be availability two weeks prior.

Two days prior, even. Except for around major holidays.

I should think you'd be fine 2 weeks out, but it is common to see trains marked as full when I hop onto the Renfe website to check a schedule for the current day or next day. I do that fairly often in answering questions here. The rail line from Madrid to Cordoba to Seville is where I've most often seen that notation, but I imagine it happens between Barcelona and Madrid as well. There are other routes with less frequent trains, and they could be problematic, too; it's just that I almost never have a need to look up their schedules, so I've had no occasion to notice sellouts.

You can easily monitor this situation yourself, and I think you should, because it seems there is a big upsurge in travel at the moment. For tomorrow there are 8 full trains between Madrid and Seville. There are 3 full trains for Saturday. There are 2 full trains for Sunday. I don't know what's going on the latter part of next week, but April 28 has 6 full trains and on April 29 there are 5 full trains and 8 others whose only seats are for anchoring wheelchairs. I'm not sure, but I think that means there's just a flip-down seat, which wouldn't be comfortable on a long trip. The upshot is that the only available train on April 29 is an MD train that takes over 8 hours to cover a distance an AVE covers in less than 3 hours.

I stick by my warning. The evidence can be seen on renfe.com right now.

Thanks...appreciate the points of view.

t

I went to Spain in March. I had bought my train tickets and one long distance bus ticket, 4 weeks in advance. Most likely you will be able to buy tickets 2 weeks in advance. There is a chance that on some popular routes that have multiple trains per day, by 2 weeks in advance, you will see fewer choices of times trains are leaving because some of the trains have already filled up or sold out. There is a chance that the lowest price tickets could sell out and you might get stuck with the high price tickets as your only choices. You might not notice the difference when you buy your tickets.

... needless to say, that will depend on when are you visiting and which routes. For many, one can purchase the same day a the station, for others, it's a bit more complicated.