What does 45mn mean? How long

UNIT OF LENGTH:
The nm stands for nanometre.
A nanometre is equal to one-billionth of a metre     1 x 10^(-9) m  .
Or 1 metre = 1,000,000,000 nm

The prefix "nano" = 1/1,000,000,000
Similarly, "milli" = 1/1,000, so 1 mm = 1/1,000 of a metre.

45 nm = 4.5 x 10^(-8)  m (extremely small unit) 



For Internet Chat:
I guess nm can mean "Nothing much" or "Never mind".

it means 45 minutes, mn is the abbreviation of minutes
it can be 45 months, mn is the abbreviation of months

actually it depends on what is it talking about

意思是四十五分鐘, 這是分鐘的縮寫
意思是四十五個月,這是月的縮寫