I can’t make any comments on a scene without seeing it, especially when it comes to something complex or adding in something like GI.
If you are using the standard GI then your settings have to be adjust for your scene specifically…there is no golden rule.
Render optimization is a pretty huge field in it’s own right and you also need to really consider if the benefit outweighs the result.
This is very true for GI…how much of the effect of your GI could be recreated with lighting.
What other considerations do you need to make for the GI?
Are there transparent objects?
Do some objects need the same number of samples as others?
Are objects animated or just the camera?
Do you need details in the GI or general light info?
As for flickering, it is avoidable with standard GI but generally at the expense of render time.
There is the option of using QMC or II, both of these options will produce flicker free results but can be slower.
Again, there are so many factors that go into the speed of a render it is impossible to say exactly what must change.