As much time as you’re prepared to give it!
This is especially true of social media, which just consumes time. Be careful with that one.
Online campaigning will be a much bigger part of this Election than it was in previous Elections. In district-based Elections, you prioritised door-knocking and face-to-face events, and dealt with emails and social media in the corners of the day (or on the bus). I think that will be basically flipped for this Election.
I would recommend making time for any face-to-face events that will bring you into contact with multiple voters – hustings, ‘meet the candidate’ events, and so on. If you have time, I would still try to do some door-knocking, but that is a lot of time for quite little return, so it can’t be what you prioritise.
Apart from that, I think most of your contact with voters will be mediated by a computer. It won’t necessarily all be “online” – in the sense that, for example, you might be asked to complete questions for a Press supplement. You’ll probably receive those by email, and sit at your computer to answer them; but the supplement (if there is one) will be printed and arrive in voters’ home in hard copy.
But there will no doubt also be a range of online surveys, voters’ questions, and social media engagement which will be purely online. This will be your main way of reaching the majority of voters, so I would make sure that you prioritise your time so that you can do it justice; and then fit in other things depending on the time you still have available.