How much time should one meditate? (annapanna or vipassana) ?
2 session of 1 hour each sitting meditation generally works well for many, but this is not the true meditative practice.
According to Lord Buddha teachings, How should a normal employee spend his/her day ?
Real meditation is from awakening to sleep. Be aware of sensation arising and passing either due to posture, bodily sensations, mental activity and thoughts, sensory experiences, etc.
As an employee you should earn your living in the right way and ethically. Meditating or other activity in office which might interfere with your duties might not be ethical.
While sitting in office, how should i sit (sometimes i try to sit as straight as possible, but after 10 minutes i am back to ordinary state)
What every posture that works for you is fine. Over rigid postures may give undue attention of colleagues which might not help.
If you are referring to "crookedness of body" in (Pacetana) Ratha,kara Sutta, this is a figure of speech referring to lax morality.
I want to know how to spend all my 24 hours in order to remain all time conscious (what exactly means conscious, suppose i am walking, should i remain aware about my hand and legs moving or my breath) ?
In short the breath and arising and passing of sensations. Generally when you are waking moving parts have more sensations, but as you progress it is good to look throughout the body.
If the sensations are pleasant you get attached if not averse. This is one thing you have to understand and be aware of. Staying in a particular posure may be present for a little while but becomes painful and the urge to more arises. When you move the new posture becomes pleasant and cycle continues. Similarly for thoughts, memories, and other sensory experiences. Regarding sensory experiences and perceive it as positive or negative or neutral, if positive you experience a pleasant sensation and you get attached to the pleasant sensation triggered by the evaluation and similarly when the experience is negative.