It's based on you individually; what may be low for one person can be okay for another:

http://www.mayoclinic.com/health/low...590/DSECTION=3

These can be symptoms of low blood pressure but, as you see, they can be symptoms of lots of other things too :-)

Dizziness or lightheadedness
Fainting (syncope)
Lack of concentration
Blurred vision
Nausea
Cold, clammy, pale skin
Rapid, shallow breathing
Fatigue
Depression
Thirst

Everyone with depression just has low blood pressure