Did a bit of Googling but there seems little to go on. Two posts stand out:

It's suggested by some that there is indeed some fixed time limit ultimately on the game:

[url=http://wiki.ultimacodex.com/wiki/Talk:Killing_Lord_British#Ultima_V_Time_Limit]http://wiki.ultimacodex.com/wiki/Talk:Killing_Lord_British#Ultima_V_Time_Limit[/url]

However, no one seems to ever be able to confirm its existence, or when it occurs, except for this mystery screenshot mentioned (which would surely be far easier to locate if this were true - however, Googling Ultima V screenshots with such terms as "time limit game over" is coming up totally dry).

I'm suspecting this is now of legendary video game rumour status, especially with the fact that the Ultima games have historically been subject to some of the most thorough scrutiny and analysis by game hackers and modders over the years, surely it would have been found by now.

Perhaps there was once to be a time limit but it was removed in development? (Time limits, especially for the whole course of the game, were a distinct rarity in RPGs prior to Fallout or so, and even most timed segments - e.g., the many in the Final Fantasy series - tended to have an onscreen countdown timer right there for you to see and clearly know how long you had till zero. Such nebulous time limits as this one were de rigeur in Sierra and Infocom *adventure* titles, but as I said - extremely uncommon in RPGs.)


It's also been suggested that there may be no actual time *limit* as such, but the game becomes progressively harder the longer you take:

https://www.omnimaga.org/ti-68k-projects/ultima-v-general-discussion/615/

This is more likely to be true (although it might be tied to party size as in Ultima IV, or perhaps to party level?)