I am not going to claim to be any expert on social media, but thought I would just share with you some of my ideas about managing social media without it affecting your writing time.
Firstly I hardly ever post something spontaneous. All my tweets and posts for my writing facebook page are actually posted by an app called Buffer. There are other apps out there – tweetdeck and hoot suite are also available – but I’ve just got used to buffer. You’re allowed to queue 10 tweets and 10 facebook posts on a free account – any more and you need to pay a subscription, but I find this is sufficient to cover a month’s worth of posts.
You can also choose the time of day and the day of the week that buffer will post for you. It’s recommended that you set tweets to appear when most people are looking at twitter – commuter time 7.45am-9am, 12.30-1 and after 9.30pm, but you might feel your target audience will be looking at twitter at a different time. Thursday is apparently a peak day for social media activity – yeah, I’ve got no idea why either. I suggest you vary it a bit to begin with, give it a couple of months and then have a look at which tweets get most views.
I try to always include a picture – and of course hashtags, but then you knew that, right?
For me, weekdays are writing time, but the weekends are for housework and admin, so once a month I spend a weekend afternoon filling up buffer and also writing a blog for the month. It’s actually quite relaxing. Also, if you’re trying to do something in particular with your tweets, it’s much easier to do this consistently if you’re creating them all in a block. The most successful twitter accounts do have a narrow remit and are consistent. 
Now, having said that, you are not going to grow a twitter following if you don’t engage. My strategy is – when I get a notification of a new follower, I go on to twitter to check whether it is someone I want to follow back. (I would really urge you not to set your account to auto follow back -there’s some weird stuff out there…) While I’m checking out the new follower, I allow myself a few minutes to read a page or so of tweets, and I’ll re-tweet or like or comment. This is something I might do in a writing break or after I’ve finished for the day. You need to work out what works best for you. I find it you allow twitter to email you notifications about new followers, that can be a prompt which sits there until you’re ready to deal with it.
I hope this helps – do let me know if you’ve got some other tips to add.