Octo's Rambles

The sending spell lets you send 25 words to someone.

The longest word in the English dictionary is Pneumonoultramicroscopicsilicovolcanoconiosis, which has 19 syllables. A google search claims people talk at 4-5 syllables per second, so let's assume that this word takes four seconds to say. That means that with all 25 words you can occupy someone's head for 100 seconds, which isn't a lot but is certainly annoying.

Now, a discerning dm might argue that you can't use this word since it's unlikely your character knows the technical word for silicosis. This is where you pull out your pet rat, and give it a fun name, something like:

Wolfeschlegelsteinhausenbergerdorffwelchevoralternwarengewissenhaftschaferswessenschafewarenwohlgepflegeundsorgfaltigkeitbeschutzenvonangreifendurchihrraubgierigfeindewelchevoralternzwolftausendjahresvorandieerscheinenvanderersteerdemenschderraumschiffgebrauchlichtalsseinursprungvonkraftgestartseinlangefahrthinzwischensternartigraumaufdersuchenachdiesternwelchegehabtbewohnbarplanetenkreisedrehensichundwohinderneurassevonverstandigmenschlichkeitkonntefortpflanzenundsicherfreuenanlebenslanglichfreudeundruhemitnichteinfurchtvorangreifenvonandererintelligentgeschopfsvonhinzwischensternartigraum

This is a real name which existed, and assuming an average of 4 letters per syllable (I'm not going to count them), is around 150 syllables, or 33 seconds of talking. This name repeated 25 times will give you around 14 minutes of talking. You could probably invent a longer name, this is DnD after all, but this gives you a baseline grounded in reality.

The sending spell has no save against it, is not constrained by distance, and to my knowledge has not way of being blocked (though I'm sure one exists).