How far in advance to buy train tickets?

It is almost never a good idea to buy advance nonrefundable tickets for departing from an intercontinental flight, unless you have a gamblers instinct. Your flight could be delayed or even cancelled last minute, cousing you to lose your tickets and then have to buy last minute tickets.

Right now, I see you could get a ticket for 19.90 EUR for tickets that would cost 137.30 EUR last minute. That's a pretty good bet for my gambling instinct.

Can you get a Rail&Fly packge with your existing ticket? if so, you can take any train after you land.

Thanks, Sam. I've read that train travel is increasingly popular throughout Europe and trains are selling out. I didn't want to get stuck in Frankfurt since I prepaid hotel in Berlin. Maybe it's not risky to just wait until I get there?

German trains do not sell out, if all the seats are occupied, you stand until some open up. You can buy seat reservations for only 4.90 EUR each if you want a guaranteed seat, which can often be a wise decision.

how much time I should allow myself to get to the train station from
the airport?

Getting to the train station is a bit of a trick question.

There is a train station at the airport: Frankfurt (M) Flughafen. It is less than a 10 minute walk from Terminal 1's custom's exit to the train station. You can get a train there that will take you to the Frankfurt(M) Haputbahnhof. There, you will want to get an ICE train to Berlin. At the Frankfurt (M) Flughafen train station there is a ticket office (with a line of people getting tickets) or a self-serve kiosks. This one ticket will cover the full trip. Your train-to-train transfer at the Haiptbahnhof should be less that 10 minutes.

Or, you can take the S-Bahn from the airport to Frankfurt(M) Haputbahnhof. The S-Bahn is under Terminal 1. You will need a sperate ticket for the S-Bahn. At the Hauptbahnhof it is about a 10 minute walk from the S-Bahn to the train platforms. You can also buy tickets for the ICE train to Berlin at the hauptbahnhof.

Unless I am going in to Frankfurt, I take the all train option.

You can do all your ticket purchases on the DB app. I recommend it. You can also get info at bahn.com. You can also see how busy DB expects your planned train to be. This can help you decide if you want to reserve a seat. The more full the train, the more valuable a reserved seat.

I know you asked about trains...

I was looking at a similar itinerary for my wife. An air ticket to Berlin was less expensive than the train and took about half the time.

At 19.90 as Sam writes, I would gamble on the train ticket. However, when I gambled on doing it,
i lost. Even though I allowed a few hours for the connection, my plane was a day late !

Thanks for the great info. Very helpful! Funny that you suggest flying to Berlin because that requires connecting flights for us and we no longer will do that due to issues with canceled or delayed flights, missed connections, and lost luggage (missing for 4 days last time!). This is why we decided to take the train. Like you said, travel can be a throw of the dice.