The ELAT is 90 minutes long and because essay writing depends on personal style, it does slightly depend. If you like to have more time to plan, then you could spend up to 15 minutes planning.
The breakdown for this would be: 10 mins reading
15 mins planning
60 mins to write
5 minutes to read through/check
If you like to spend more time writing, i.e. taking breaks during essay writing and reading as you go, you could spend 70 minutes. The break down for this would be:
5 minutes reading
10 minutes planning
70 minutes writing
5 minutes to read through/check
As far as planning the time goes, stick as closely to one of these plans as you can-running out of time would just be a silly and easily avoidable mistake!