Mark is right. Capacity is measured in Amp-HOURS.
How much current you draw matters, but how long you draw that current, and over what duty cycle also matters.
How many amp hours did you use? That depends.
First off, the amp hour rating of a deep cycle battery (Lead acid chemistry is assumed here) is usually determined by supplying a constant 0.05C load current for 20 hours. So if you have a 20 Amp hour battery, that means it should deliver a 1 amp continuous load for 20 hours before it is depleted.
Note that the term "depleted" is a very relative term. From the battery manufacturers standpoint, depleted is when you have reached the maximum stated depth of discharge, which usually occurs at a voltage of 10.5 volts for a 12 volt battery (Lead acid chemistry). But your radio wants to see 13.8V +/- 15%, so for you the battery is depleted when your rig will no longer operate properly, which would be at a voltage of 11.7 volts.
But you're not going to draw a constant current. Your current draw will vary between transmit and receive.
So, how many amp hours DID you use?
Let's take your FT-450 as an example. Say it draws 6.6 amps on transmit (at 30w output), and 1.5 amps on receive.
You transmit for 13 seconds, and then listen until it's time to transmit again. For the sake of this example, let's say you transmit once every 5 minutes, or 12 times per hour.
So over the course of an hour, you'll transmit for 13 x 12 or 156 seconds, and you'll receive for the the remaining time, which would be 3600-156 or 3444 seconds.
156 seconds is 4.3% of an hour, so you're receiving for 95.7% of an hour.
6.6 amps * 4.3% * 1 hour = 0.28 amp hours on transmit, and 1.5 amps * 95.7% * 1 hour = 1.45 amp hours on receive.
So in this example you would have consumed 0.28 + 1.45 = 1.73 amp hours during that 1 hour time period.
And you can keep doing that, until the battery voltage drops to 11.7 volts, however long that takes. You will have to discover that empirically (trial and error).
If you transmit more and receive less then your amp hour consumption will rise, and your battery will deplete faster.
If you transmit less and receive more than your amp hour consumption will decrease, and your battery will deplete more slowly.
Hope this helps, and makes sense.
73 de N8AUC
Eric