We all know pretty much how to do time expressions using 'tenpo' and 'la' and whatever else is relevant, but the usage has not been codified beyond a few examples, There are also some time concepts that are not obviously covered by what we have so far. Further, there is a desire (maybe even a need) to shift some time expressions from the condition slot to the preposition slot or to use complex time expressions as modifiers. This an attempt to spell what I think we all agree about and to point to some few hints of a way forward.
tenpo la sometime(s), not never, not always, generally not even regularly
tenpo lili la rarely, occasionally
tenpo mute la frequently, often, generally
tenpo [nanpa] la [nanpa] times (with pini or kama to say if all occurrences are in one direction or the other)
tenpo suli la for a long time, hence: ? [add awen? aa}
tenpo lili la for a short time, briefly X aa
tenpo pini la (at some time) in the past, past tense
tenpo kama la (at some time) in the future, future tense
tenpo suno/pimeja pini la yesterday/ last night, on a past day (also with (tenpo) sike suno, (tenpo) sike mun and tenpo esun)
tenpo suno/pimeja kama la tomorrow/ tomorrow night, on a day in the future
tenpo suno pini tu wan la three days ago ? (similarly for pimeja and kama and other numbers)
tenpo suno tu wan pini la after three day, three days later ? (etc.)
tenpo suno tu wan la for/during three days ? (etc) aa
tenpo suli pini la a long time ago ? (etc.)
tenpo la for a while X aa
I think I am forgetting some time expressions here, but this is as far as I can find even hints of usage.
For prepositions there are not a lot of usage, but the pattern seems to be x la goes unchanged to lon x for those that point to a particular time or times. For those involving a duration, the obvious thing is to replace the tense word (pini or tawa) at the end with a preposition at the front, but which one gets tawa and which tan is open to a long argument. And the "for a" cases don't seem to fit anywhere, though you will note a suggestion above.