My 5 years will complete on 12/03/2018. What the earliest I can apply and what is the end date I can apply?
Within 28 days before meeting the requirement. You don't state the date your visa was issued and the date you entered the UK.
What is the processing time for premium service and postal service?
Premium service if for on the day decisions (no guarantee though of a same day decision). Postal applications can take a few weeks to a few months, depends.
Some members have reported that the BETA online form is no longer available. You can try, if it doesn't work, you will need to complete the paper form.
What are the details employment letter should have?
The form and guidance notes will tell you what you need. You need an absence letter for all absence in the 5 years and you also need a letter stating you 'are required for the foreseeable future and that you will be paid at X salary for the foreseeable future'. Presumably you meet the salary requirement for ILR?
Is life in UK test mandatory for Tier 2 general?
Yes. Why do you think it would not be??