Hi Estellaa,
I regard anything below 4.0 as requiring action. My normal (and preferred!) remedy is jelly babies. I will take between one and five depending upon how low I am. I will also aim to take another BG reading about 30 minutes later with the hope that my level has risen above 5.0, if not then I will swallow another jelly baby or three. I do this after 30 minutes because my reaction to the fast acting glucose in the sweets is effective but a little slow, for other people 15 minutes may be a sufficient pause before doing a retest.
As Northerner and Sheena have said it does depend on what I am planning on doing when I get the low reading. For example if I am about to drive I will raise the level at which I require action to 5.0. If I am about to go for a run then 6.0 is, for me, a safer lower limit.
When I do react to a low BG it can be quite varied. Normally I become very tired; often a lot of sweating; sometimes a headache; sometimes it affects my mood; sometimes I become confused and/or incoherent.
A further issue I do have is that when I get the above symptoms but have not taken a BG reading then quite often my initial reaction is to go into denial that it is happening. It can then become a bit of a challenge to get myself to accept that it is happening and take appropriate action before reaching the point where I need some help from someone else to get it sorted. I think what has contributed to me having this denial response is being taught when I was first diagnosed that if I had a hypo I had done something "wrong". Now that I know that I am not necessarily to blame for a hypo and they can happen to even the best controlled insulin dependent diabetics - just read some of the other hypo related posts on this forum - I am more willing to accept that one is happening to me and deal with it quicker. Afterwards rather than feel guilty about them I now just review what may have been the cause with the aim of making one less likely to happen in the future.
One problem I have had in the past is that I have had periods when I have had quite regular hypos to the point where my body became more accustomed to low blood sugars and I lost much of my awareness and symptoms that one was happening. You can eventually get the awareness back but when it is reduced it does mean having to be particularly careful to do a BG test before doing anything like driving, exercising or even just going for a walk. This can be a real pain at times. I would therefore strongly recommend that you do your upmost to avoid regular hypos and that you do take a quick remedy for any readings below say 4.0 so that you maintain your awareness and symptoms, even if those hypo symptoms are unpleasant.
Hope that helps!