"
One of my colleague told me that it would take no more than 8 months for the green card to be approved for a Nepali citizen. But it would take longer for lets say, an Indian or a Chinese person. Is that true? "
It depends on which employment based category u r filing..Priority date for EB2 is always current for Nepal..For EB3 priority date for Nepal check US visa Bulletin..
These days there is a Back log in Labor..
If u r lucky , you may have GC in 6 months after filing Labor..USCIS is unpredictable..But during these times i highly doubt that GC will be Approved withing 6 months..Realistic scenario is 8-1 and half years...I am assuming that You will be filing in EB2 Category..
"I will soon be out of status in few months and seriously need a suggestion"
During GC filing you have to maintain Valid Status all the time..So filing GC straightaway is not going to work for u..
"Since a student visa doesn't allow "dual intent" as in H or L visas, it seems that filing a green card application - a clear indication of immigrant intent - would terminate one's F1 status immediately"
It doesn't terminate immediately as u have shown immigration intent..But if Your I140 is denied You wouldn't be eligible for F-1 status as u have shown immigration intent..That's why PPL suggest applying for GC while u r in H or L visas as it is dual intent...