## Is your feature request related to a problem?

#4947 added a new query to the backend – time to convert. It works well, just one point is kind of rough: bin width and count.

## Describe the solution you'd like

Currently the number of bins is hardcoded. Ideally we should have two options:
- return a user-specified number of bins,
- otherwise infer a reasonable number of bins using some algorithm (auto option).

@alexkim205 [suggested](https://github.com/PostHog/posthog/issues/4855#issuecomment-872423877) the [Freedman-Diaconis rule](https://en.wikipedia.org/wiki/Freedman%E2%80%93Diaconis_rule) for determining bin width (and interval based on that), but unfortunately from my testing its formula would not be the most practical – it's great for keeping a pretty constant "smoothness" of the histogram esp. with normal distribution, but we face some additional constraints that make it less than ideal:
- we can't have histograms too dense visually in the UI, from my testing a practical limit is 60 bins (above that it gets too dense),
- our data generally doesn't follow much of a normal distribution.

Clamping may help though, or maybe a different formula may work better outright – I'll be testing this.

## Additional context

Issue pertaining to the experience of time to convert: #4855

#### *Thank you* for your feature request – we love each and every one!

posthog/models/filters/mixins/funnel.py
@cached_property
    def funnel_window_days(self) -> Optional[int]:
        _days = int(self._data.get(FUNNEL_WINDOW_DAYS, "0"))
        if _days == 0:
            return None
        return _days
    @include_dict
    def funnel_window_days_to_dict(self):
        return {FUNNEL_WINDOW_DAYS: self.funnel_window_days} if self.funnel_window_days else {}
    @staticmethod
    def milliseconds_from_days(days):
        milliseconds, seconds, minutes, hours = [1000, 60, 60, 24]
        return milliseconds * seconds * minutes * hours * days
    @staticmethod
    def microseconds_from_days(days):
        microseconds = 1000
        return microseconds * FunnelWindowDaysMixin.milliseconds_from_days(days)
class FunnelPersonsStepMixin(BaseParamMixin):
    
    
    @cached_property
    def funnel_step(self) -> Optional[int]:
        _step = int(self._data.get(FUNNEL_STEP, "0"))
        if _step == 0:
            return None
        return _step
    @include_dict
    def funnel_step_to_dict(self):
        return {FUNNEL_STEP: self.funnel_step} if self.funnel_step else {}
class FunnelTypeMixin(BaseParamMixin):