Hey Optimistic, welcome first of all!
Admittedly I do not check this website much at all, but logging on and seeing your post - I thought I would do my best to answer your questions. Most of these can be answered through their (SAMFS) website, and searching through the forum.
•Has anybody heard any news of a 2018 recruitment drive?
There is only scattered information about a recruitment campaign for this year. If you went to the SAMFS Open Day in November, they had a stall about recruitment. They only said 'opening soon'.•When can we expect applications to open up/what time of year has recruitment usually started?
There isn't any real set formula... Recruitment depends on many factors. The past two drives have opened in the second half of the year. •How many people/squads did the 2017 recruitment drive produce?
Firstly, there was no recruitment held in 2017, all squads that went through were from the campaign that opened in September 2016.
Each squad is 18 people, the fourth squad from the Sept '16 campaign is commencing 22/1/18 (according to the other forum board). Speculation going around that a fifth squad will be selected to proceed.•Who has been through the process and what was your experience?
Stacks of people here have been through, each getting to different stages. As it says on the website, it's quite competitive.And a personal question from me is:
How long between each assessment did you have to wait? (I.e. did you do your PAT1 and 2 together, separate, the day after your interview?)
This is a factor that can't really be calculated, sorry. Jump on the website and you can read the process in greater detail. The PAT1 and 2 are not run on the same day. In short the process is: online app> psych/aptitude test > pat1 > interview > pat2 >criminal history check > medical. So you may have to budget for up to 5 trips, should you make it through. 
Advice is to get to know the process well, what is expected, and prepare.
Cheers.
