A 6-hour interval is no problem. I uploaded https://skia-review.googlesource.com/c/buildbot/+/177320 for that.

Unfortunately, we don't currently have support for only landing rolls within a given time window. I'm not opposed to adding something like that, but it raises some questions:
1. Is this best effort, or do we try to guarantee that we only land rolls during the given window?  The latter is tricky because we're at the mercy of the commit queue, and we'd need to actively cancel rolls when the window closes.
2. The simplest thing to do is to not upload new rolls outside of the window, and just specify a shifted window to account for CQ time.  The problem there is that we can't make any guarantees; CQ backup could still cause a roll to land in the middle of the night.

The following revision refers to this bug:
  https://skia.googlesource.com/buildbot/+/25dc8496be41c8dd396fa0e6bf5f3038b5718995

commit 25dc8496be41c8dd396fa0e6bf5f3038b5718995
Author: Eric Boren <borenet@google.com>
Date: Thu Dec 13 13:21:35 2018

[autoroll] Roll Fuchsia SDK -> Chromium every 6 hours

Bug: chromium:914595
Change-Id: I7fba6e690b695c34f810890f319a5d3f9132511e
Reviewed-on: https://skia-review.googlesource.com/c/177320
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Eric Boren <borenet@google.com>

[modify] https://crrev.com/25dc8496be41c8dd396fa0e6bf5f3038b5718995/autoroll/config/fuchsia-sdk-chromium.json

Pushed the above to the roller.  Because of the way the throttling works, the next roll attempt will be uploaded in ~6 hours.  If you want, you can push the "unthrottle" button here: https://autoroll.skia.org/r/fuchsia-sdk-chromium-autoroll

Re #3: Thanks!
Re #1: Best effort would be absolutely fine; the purpose here would be to reduce the wasted resources + spamminess of autoroller failures that occur out-of-hours.  Having a single roll scheduled in-hours that actually lands overnight, as per your example, would be acceptable.

Marking this bug Started; if the out-of-hours feature request is best tracked elsewhere then let me know and we'll close this out and file a separate bug for that. :)

I filed a bug for the new roller feature here: https://bugs.chromium.org/p/skia/issues/detail?id=8627

I have several other things that I need to get done first, unless this is very high priority for you.

Re #5: No, this is nice-to-have. FWIW the Fuchsia SDK will be published less frequently in future, at which point I think we should switch the auto-roller to check for updates hourly, to reduce the delay in picking up new builds.  Shall we keep this bug open to track that?

Alternatively, we can just turn off the throttling and let it roll as soon as a new version is available.  I have no preference whether it's this bug or a new one, or just a ping when you're ready.  The change is a one-liner, so it should be quick and easy.