I tried that out, 1 week straight of going swimming with cath and gabby and smoking with gabby yet no new progress. :/
Progress is until the limit is reach for every character, I though 25% (could be 20%), so every 25%. IF you have 5%, flit/chat with the correct answers, so you get friendship points, until you reach 25%. Then new events will be triggered, and when you get 26% for example, you can try to chat with them until you reach 50%. If you reach 25%, chatting has no further effect, until some new events have been triggered.
Mostly those new events will come up as quests or calls within the next 2-3 days.
On the left you have your tab with open quests. You can search for the "paradise lust walkthrough", it shows for every girl, what you are missing, or what you need to do for every quest, there you can find your current quests.
You can try to buy paint, so you can paint the girls. You can try to collect the flowers, to give as a present to the girls.
There is not much more to it. I cheated a little bit, by using the debugger and use get-items to increase the $ex coins, working all day was too tedious, when you needed to buy clothing for all the girls...
Without the walkthrough I probably would have been lost too...