It seems it's any portal.

I'm at 10 days at the moment, and on weekends I'm not following my usual daily routine, meaning I'm only walking to a closer portal not en route to work. Still counting.

Max 36 hours, any portal, only successful hacks count

NOTE: These "rules" are not officially supported and may change at any time!

The only rule that is confirmed by Niantic is "max 24 hours per hack", although they have admitted there is a little bit of grace, but they never confirmed what that grace is.

I just did some testing on this, and the rules appears to be as follows:

• You gain Sojourner medals for achieving a Hack Steak of 15, 30, 60, 180 and 360 days
• Hacking on any portal counts
• Only successful hacks count; failures due to full inventory does not count
• Max 36 hours between hacks to maintain the Hack Streak

So this is the series of hacks I did, and the corresponding Hack Streak counter

• Sunday, 20:12, 0 days
• Monday
• Tuesday, 08:10, 1 day
• Wednesday, 20:08, 2 days
• Thursday
• Friday, 08:06, 4 days
• Saturday, 20:04, 5 days
• Sunday
• Monday, 08:02, 7 days
• Tuesday, 19:00, mistaken failed hack (inventory was full), 7 days (should have been 8)
• Tuesday, 20:04, successful hack, 0 days

It might be worth noting that the Hack Streak counter said 8 days in the seconds just before the Tuesday 20:04 hack, and was then 0 directly after.

Notes on the Hack Streak counter

• The Hack Streak counter is unreliable. It can say the Hack Streak is still on when it has in fact been reset
• The time of day of the very first hack only tells when the hack streak counter will tick over after on a successful hack
• The tick-over time is UTC, so if you started at 13:00 standard time, it will tick over on 14:00 in daylight saving time
• The counter may tick over despite you not having hacked for that period, so you cannot use that as an indicator to recall if you have hacked successfully recently or not