There's a few different ways you could approach this. I'm assuming you're in Lighthouse Studio but correct me if you are a Discover user.
I think any solution would probably involve the PageTime(StartPage,EndPage) function so you can calculate the time spent on the first two tasks. Lighthouse will show you the page number at the bottom of the Write Questionnaire window if you click to highlight a question.
I would then probably put a text page with the warning in between the second and third CBC task, and then your skip logic could be to skip over that warning if the page time function returns greater than the value you choose.
For terminating you could do the same, put a preskip on the first question after the CBC exercise that terminates if the page time from the first page to the last of the exercise is below your threshold.
You could also use a quota to just make a flag. Put a quota between the last CBC task the next page where cell 1 has qualifying logic of the PageTime() function being less than some value, and cell 2 could just be Always Qualify. Put the cell limits to 9999 so they will never fill up and that will automatically assign someone a 1 or a 2 for that variable so you have a flag.
Please be careful with timing, requiring 15 seconds for every conjoint task seems a bit long to me personally, especially if there are levels that make it easy for respondents to simplify the task (I don't buy Brand A, I won't spend over $X) etc. Take the survey a few times with a stopwatch in hand and make sure you feel comfortable in your decisions.