After the wave hits on Miller's planet, Cooper asks Brand "How much is this gonna cost us?". To which Brand replies "decades".
How does the wave and resulting damage from the wave cost them decades when the entire wave scene and post wave scene is one continuous shot that only lasts a few minutes? Is there a break somewhere in the scene where at least a couple of hours could have gone by?
He isn't talking about the wave, he's talking about the time it'll take to drain the engines of water and return to space, noting that they're in a gravitational well so large that time on the planet is passing much faster than on the Endurance:
The novelisation explains it a little better than the script;
“How bad?” he asked, thinking that he most likely didn't want to know. “Every hour we spend on that planet will be maybe…” She did the mental computations. “Seven years back on Earth.”
and
“How long to drain, Case?” he asked.
“Forty-five to an hour,” the robot informed him.
Cooper shook his head and uncoupled his helmet. The cabin was pressurized. Everything smelled wet, but it didn't smell like seawater or a pond. It smelled like distilled water that had been dumped on hot rocks—a mineral scent, but not salt.
“The stuff of life, huh?” he said. “What’s this gonna cost us, Brand?”
“A lot,” she said. “Decades.” Her voice was flat.
Cooper felt like he couldn't breathe. Decades. Tom and Murph were adults already. How old? It seemed impossible. He rubbed his face, trying to comprehend it. He watched the wave go, knowing there would be another, and soon.
As it happens, in the end it doesn't actually take them as long to leave the surface as they first feared, evidently due to Cooper's innovative idea of blowing air through the engines to clear the water.
I took Brand's statement to mean that the trip itself wasted decades of Earth time. Their Ranger had just been damaged by the wave and they lost one crew member, and on top of that, they realized that Dr. Miller had died shortly after landing (only a few hours before by the planet's time) and the planet was inhospitable. They should have been able to tell that not much time had passed on the planet (see here) since Miller had landed, and Cooper had suggested beforehand that they visit the other planets first because of the severe time dilation. So the trip was fruitless, and it took them 23 years.
It's due to the time dilation effect of being so close to the black hole Gargantua.
They say before they go down to Miller's Planet that one hour equates to seven earth years. Thus, when they get stuck there (for several hours - it's not actually one continuous shot, there is a break in there) it uses up several multiples of 7 - thus, decades. When they get back up to the ship, they discover they have been there for 23 earth years (or, they've been gone for a little over 3 hours).
It's 7 yrs per hour, as they calculated - however, counting from descent to arrival back at the Endurance (remember Endurance stayed in safe distance, not in the immediate orbit. Time distortion started for them soon after leaving). So: descent, landing, wave hitting, engines flooded. CASE estimates it will take 45 mins up to an hour to drain. When the next wave is approaching, the engines still need 2 minutes to drain, so around 40-60 mins have passed. Cooper sparks them and they get back. Landing, time on planet and getting back to their ship not in immediate orbit - this all took them a bit more than 3 hours in total.
Brand says "I don't know. Decades" as a desperate short reply immediately after CASEs estimation, in that moment they all don't know for sure how many hours it will take them to get back to the Endurance (will the Engines go back online immediately when drained or are there any other drawbacks ahead?). All they know is they can't start right away, it will be app. an hour at least before liftoff and the entire trip therefore longer than estimated.
Its Easy and Clear to understand this way.
Romilly Says: "If we are talking about couple of hours", I will study Means: Their to and fro journey from Endurance to Miller's planet is approx 2 hours. = 14 years (approx) on Earth
Plus 15 minutes of giant wave hitting them = 1.5 years (approx) on Earth
Plus 1 hour of water drain from Ranger = 7 years (approx) on Earth
14 + 1.5 + 7 = 22.5 years approx on earth. (approx is justifed)
You need to take into account the fact that it would take over an hour to descend to the planet and another hour to ascend from the planet and track down Endurance during orbit. A very broad and general understanding of basic space flight is required in order for this to make sense. Getting to a fixed point in space is trickier than movie editing makes it seem. Of course they were on the planet for longer, it takes 5 minutes just to get your gear on and open the hatch of the door.
