Yusuf: Brain function in the dream will be about 20 times of normal.
When you enter a dream within that dream, the effect is compounded.
The only calculation is multiplying by 20.
The flight lasts for 10 hours, so in the first dream you have (10 x 20 =) 200 hours. In the script, however, they quote one week which is only 168 hours, so we might need to assume a bit of time at the start of the flight to drug Fisher and get set up, and time at the end to clear the gear away.
In the second dream, we multiply again, giving us 20 weeks - again someone in the script didn't get their maths right because the script talks about six months, whereas 20 weeks is only about 5 months.
In the third dream, six months multiplied by 20 is indeed 10 years.