How long do people allow for day trips to other cities?
While in Madrid, I plan on taking a train out to Segovia for a day and Toledo for another day. While in Seville, I plan on taking a train out to Cordoba for a day.
I see a lot of suggestions from people to buy your return ticket in advance because the train tickets can get sold out, which means I'm going to have to plan my time in these cities beforehand. How much time should I allow?

The train tickets are usually much cheaper if bought in advance on the RENFE website, at least for the high speed AVE/AVANT trains between Madrid and Toledo (some 30 mins) and between Sevilla and Córdoba (some 50-55 mins). Segovia would be a little less than two hours.

In all cases, I would have gone after breakfast, planned a solid Spanish lunch (the traditional three course meal from about 1.30 is often a great bargain in most restaurants) and returned "home" for dinner or a lighter meal in the evening from about 9-10 pm.

In Córdoba there are several fine bodegas/tavernas in the area around Plaza las Tendillas and Plaza de las Correderas just some 200-300 meters and 5 mins walking north of the main tourist attractions.

Here you'll find great local places on every corner. Just had some excellent meals in a couple of them a few weeks ago. Taberna Salinas comes recommended in the Michelin guide and we were perfectly happy after two raciones, a bottle of wine and a couple of brandys for some 35€. http://www.tabernasalinas.com/
Old Taberna San Miguel is also just off central Plaza de las Tendillas with lots of cafés and a much more local atmosphere than around the Mezquita where most of the tourists end up. http://www.cordoba24.info/tavernen/h...miguel_en.html

train travel from Madrid to Segovia is only 35 min ( mostly through tunnels)., but it takes time to get to the train station in the north of Madrid, and from the train station in Segovia to the center of town.
I have not taken the bus, but it might be a good idea...an hour from center to center.
Toledo and Segovia are very popular day trips, tickets do sell out for certain times of the day.
Some posters prefer staying the night.... some don't.

Get an early start because lots of places close up for lunch.
Take the bus especially to Segovia and Toledo!
Trains don't need to be booked for such short trips but you need to know the train stations are outside of the cities and so you will need to wait/take a bus into the cities.Trains also cost more!
The Segovia bus leaves you walking 4 mins to the aqueduct and the great tourist office.In Toledo you get dropped off right in the main square!
Do lunch there and then relax on your ride back to Madrid and be ready to have more adventurea!

The problem with not buying your tickets in advance is that the train you want to take could be sold out, which means you would have to take a later train, cutting into your time to visit Toledo, Segovia and Cordoba. It's the same for the return, so always buy the roundtrip ticket and plan your day the way you want to and not leave it up to chance, unless you don't mind wasting your time sitting around the station.

How much time you allow on a day-trip will depend on what you want to see and do, but personally, there never seems to be enough time if you want to have a relaxing lunch and enjoy the day.

For short trips I feel there is no need or real advantage to buy tickets in advance, they are a few euros and that is it. Usually there is only one class, if you have a choice take always second class, sometimes a more expensive train might do the short trip you are looking for usually it is more expensive no need to take it.
Remember taxi charge a small supplement in and out of train stations. Buses are good options check were the stations are located and choose the ones more convenient to your hotel.