It depends on how much you write per post and what the content is. Really, the question you asked is about the hardest one in the world to answer. Let me give you an example.
Say you posted a brief paragraph about .... practicing your bow to increase your bow skill. There's twenty posts total, but each one has just about two or three lines... I might award that an XP... maybe two if its content heavy meaning it has a lot of detail on how to actually improve your firing or you used your bow a lot.
Now... say you posted one or two posts, but they were lengthy and had content you could add to the wiki as a new article on a new skill or lore. I might be just as apt to give you a full lore for that or say two or three XP right off the bat. If you posted say ten posts heavy on content, I might actually give you a full 5 XP for it.
It is just so hard to answer this question. We don't have a X number of posts per training thread with a word count of X requirement. Also, someone who doesn't write so well but really made an effort on spelling, sentence structure, and showed a shining improvement or an excellent use of English when they were say native to Timbuktu - I'd probably give them an extra point or two just because I didn't have to suffer through their usual misspelling or horrid grammar... but I'd also tell them that!
If your doing an actual training thread, I'd guess it would need to be longer than say your PC researching for a lore... but again that depends on what your usual post length and detail is. Also, say your suffering a horrible injury like you and your table saw tangled over the weekend when you were irl practicing your home improvement DIY warrior skills and you now have more stitches than flesh left on two fingers (that happened once to a friend of mine... he's fine now but he uses the guards ALWAYS and appreciates his pinkie and ring finger a whole lot more) we do take into account valiant effort in the face of IRL and cut a lot of slack for it.

Now, I'm sure you found this not so helpful! Maybe the others can pipe in.

My best advice is jump into your training threads full throttle and just see what comes out of them. I'm always open to the whole 'Goss, I want 5 XP for this... where am I at in this thread and do I need to do more still?'. Im very opinionated and often have threads read in my areas so I can answer right away - and will say "Hey, your X amount of XP into it... you need to do Y to get to your goal. Here's a suggestion or two...." or "Woah.. you passed the 5 XP mark thirty posts ago....". I honestly wouldn't worry about length and just start writing, have a good time, and let your PC learn some new things or improve some old skills.
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*whispers* I always also give extra credit for PCs that struggle, have trouble by not always brilliantly succeeding the first time, or write humor into their threads. Cause I have a soft spot for the underdog, the awkward, or the hopeless. I've been there irl, and know how it feels.

Jen