what does "i couldn't have been gone more than an hour, an hour and fifteen"mean?
“Then we’ll take it away,” he promised. “So go through it with me.”
“I went out, walked to the village, spent a little time in the gift shop. I bought an ice cream cone, came home.”
As she spoke she poured the lemonade over ice, added a plate of crispy cookies to the table. “I couldn’t have been gone more than an hour, an hour and fifteen.”
“Did you lock the doors?”
“Yes. I’ve been careful, or mostly careful, about that since the break-ins at Bluff House.”this sentence is conflict, isn't it? the former part=she didn't leave more than an hour, that is to say, she must be back within an hour, then what does "an hour and fifteen"mean?

Question

what does "i couldn't have been gone more than an hour, an hour and fifteen"mean?
“Then we’ll take it away,” he promised. “So go through it with me.” 
“I went out, walked to the village, spent a little time in the gift shop. I bought an ice cream cone, came home.” 
As she spoke she poured the lemonade over ice, added a plate of crispy cookies to the table. “I couldn’t have been gone more than an hour, an hour and fifteen.” 
“Did you lock the doors?” 
“Yes. I’ve been careful, or mostly careful, about that since the break-ins at Bluff House.”this sentence is conflict, isn't it? the former part=she didn't leave more than an hour, that is to say, she must be back within an hour, then what does "an hour and fifteen"mean?

Answer

She is saying that she wasn't gone from the house for long - not more than an hour, but then qualifies that with 'an hour and fifteen' - conceeding that it may have been a bit more than an hour but not much more.  This use of 'fifteen' suggests an American speaking.  A British English speaker wouldn't use that expression.  We would say "an hour and a quarter', or, would add 'minutes' after the 'fifteen'.

Answer

The modal verbs "must" "can" and "could" are used in the past (+ have + past participle) for logical deduction.

"Where are my house keys?  Oh no - I must have left them on the bus!" [positive statement]

"Where are my house keys? I can't (or "couldn't") have left them in the office because I remember taking them from my desk when I left." [negative statement]

In this case, I guess there was an incident which took place when the speaker was away from home.  There was a discussion about it.  The speaker logically deduced that she could not have been away for more than 1 hour to 1 hour and 15 minutes.