Hi hi!
We utilize a shared splash when there's a group of skins related to one thematic coming out in the same patch. Some examples of that would include Warden, Marauder, last year's Pool Party, as well as the upcoming wrestling skins. The splash has more potential to be more creative and more dynamic as a whole - it's more of a piece of art (and IMO makes for a cooler wallpaper, but that's not part of the decision. ;3)
In most cases, we try to give 975+ RP skins their own splash when possible. Last year, we felt the Pool Party skins would work well together and made for a more interesting, cohesive splash, and some of you guys agreed, and some didn't. It's feedback we've taken and have considered for future splash art.
As far as continuing skin lines, it's a matter of strong thematics that we feel could work on other champions, or thematics that players gravitate to or really enjoy - Pool Party, Snowdown, PROJECT, for example. PROJECT: Yasuo was the first PROJECT skin and players loved the idea, and even went as far to create more PROJECT concepts for different champions.
Speaking in terms of release cadence over time, it really depends on the other content we're releasing in the year. There are some events we can't move skins for - holiday stuff like Valentine's Day, or if we did more Pool Party, we'd want to release it somewhere in the summer months.