I would argue that you can't continually question the target using Suggestion. The course of activity the target must comply to needs to be described in a sentence or two. The target then acts out the activities you described to the best of its ability, but is not subject to any additional commands. Saying "answer my questions truthfully for an hour" is not strictly describing one course of activity, it's describing multiple possible courses of activity that branch out depending on the answers.
You can, however, get a truthful answer to one question that you can describe in a sentence. The suggestion would be "Answer this question truthfully: ...". It is clear what the target must do and the suggestion itself sounds reasonable (though that's debatable, depending on the question). When the answer is given, the spell ends.
I'm not quite sure that my interpretation of a "course of activity" is the correct one, though. I could personally find reasoning either way. But for this specific case we already have a similar spell, the Zone of Truth, so we can approach the issue from a balance aspect.
Let us compare the relevant characteristics of the two.
Suggestion (used to force truths)
- 2nd lvl enchantment, 1 action, 30ft, Concentration, Wisdom save
- lasts for 8 hours, or until the request is completed
- one target
- must describe the action in a sentence or two
- the spell fails if "the action doesn't sound reasonable"
Zone of Truth
- 2nd lvl enchantment, 1 action, 60ft, no Concentration, Charisma save
- lasts 10 minutes
- all creatures in a 15ft radius
- creatures can't lie, but they can try to avoid saying the truth they don't want to tell
First question is, is Suggestion strictly better than Zone of Truth in this respect? I would argue not. They target different saving throws, so there's even a merit to having both spells prepared (if you really hate being lied to). And while the Zone of Truth can be worked around if the creature is intelligent enough to avoid saying the sensitive truths, the Suggestion's application is limited to the truths that aren't unreasonable to blurt out (unfortunately, what is considered reasonable is rather subjective).
Keep in mind, though, that Suggestion is rather versatile. The amount of utility Suggestion brings to the table could make it a better choice than Zone of Truth in almost all situations, which would defeat the purpose of Zone of Truth's existence. If your DM feels this way, it would be best to rule that Suggestion shouldn't be used to force truth out of a target. The Zone of Truth can affect multiple creatures, though, so it still has its uses.
Should the DM allow it to even be possible, the question becomes how to rule the specifics of the spell. Considering the two spells should be of a similar power level, and taking note of the advantages and restrictions of Zone of Truth, I would rule this use of Suggestion one of two ways:
- You can get one definite truth from your target.
This is the same as what I described in the first part of this answer. "Answer this question truthfully" sounds reasonable enough, and while the revelation of the truth might bring the target into harm, it is not directly putting itself into harm. As an example, if a target would confess to a crime as a result of this effect, it can still try to escape, reduce its sentence, bribe or otherwise avoid punishment. After the request is completed, the spell ends, so no more truths would be forced.
- You can question the target for 8 hours, but you can't ask it to answer questions that would (although indirectly) bring it to harm or otherwise be unreasonable to answer.
The request would be "Answer all my questions truthfully", but it would be limited by the spell's effect. This is somewhat similar to the restriction of Zone of Truth in the way that it won't be necessarily easy to get the truths you're looking for. As for the other aspects of the two spells, you're basically trading a multi-target, Concentration free, 10 minute effect for a single-target, Concentration required, 8 hour effect. Though I doubt you'd ever need more than 10 minutes to get the truth you want if it's possible to gain.
Alternatively, there is a third ruling you could make, which in my opinion is overpowered:
- You make the target give you definite truths for 8 hours.
In comparison to my first ruling, it's the difference between a villain telling you how to get out of a trap he placed you in, and him telling you his entire backstory, all his evil plans, how to nullify everything he's ever done and what he had for breakfast this morning.
In comparison to my second ruling, it's the difference between "We can get some good truths, we just have to be careful about what we're asking and how the questions would be perceived as reasonable." and "Tell me the easiest way to remove the protective spell that you have on you, the keyword to deactivate your mechanical guards, and the elemental effects you're not immune to. But don't worry, we won't kill you afterwards. We wouldn't.".