Hiya!
"When it feels right".
Seriously. I've been thinking about how to describe my style of DM'ing and am coming up all nebulous and wishy-washy.
Basically, if it's an "obvious mechanical game thing" (Saving throw, To Hit roll, Damage roll, etc), 9/10 we just roll as the rules indicate. I do use rolls on my end when I can think of multiple likely/possible events, assigning various chances, and rolling a die. For example, if the PC's have semi-successfully made some rolls to hide/evade a pursuit by a crazed owlbear, I may think about how determined the owlbear is at getting them, if it is used to the area it or the PC's are in, the weather, time of day, if the PC's are injured, and anything else that comes to mind that I think would influence its decision. I may end up with "1d8; 1-3, keeps following them correctly down the left path; 4 - 6, incorrectly follows them, going down the right path; 7 - 8, gives up and goes home".
As for "Ability checks"...all the time, but it's up to the Player to tell me if he thinks he can use a skill in it's place. In other words, if the PC has Survival with a +6 (with only +2 of that being from his Ability Score), it's up to him to say "I have Survival...?" so I can say "Yup, use that", or "Nope, not in this situation". Even if the situation seems "obvious" from the players perspective, they accept my Yes/No because they generally trust me to know more about what's going on. (e.g., maybe the owlbear is actually possessed by a demon spirit and has some extra-sensory perception thing.... 'detect life' or 'sixth sense' or something). I rarely get arguments, and it's usually enough to smile and say something like "Well, if it was a
normal owlbear, sure...". Yes, it gives some stuff away, but at the same time it definitely peaks my players interest.
Anyway... as for "when" to roll? I can't figure out how to articulate it. It's mostly a "gut feeling" (experience kicking in, is my guess). I use it a lot for pacing and tension. Making lots of rolls in succession is a balancing game; it both builds tension, but too much of it can destroy the pacing. Having a roll made by a player once ever 15 minutes slows down the pacing. If I also focus on description and role-play, it can build tension really well. Then, a series of quick dice rolls in succession, with quick descriptions and "using the players imaginations against them" (so to speak), changes the pace and tension. As I said... wishy-washy and hard to describe. *shrug* It's just experience as to when/how much to use it all I guess.
So, "mostly never, except when it's all the time".
^_^
Paul L. Ming