What would hold up in the court of law?
I'll get back to you - see below.
Is there any sort of implied expiration date for a contractor's completion for medium size contractor jobs (< $10k)?
Yes, if a contract is silent on when something is required to be done than it must be done within a reasonable time.
It may be that a month's delay is reasonable or it may not be.
Will my verbal complete-by date hold up in lieu of any written complete-by date?
The parol evidence rule prevents you from introducing evidence outside of the contract that contradicts or clarifies something that is in a written contract. However, such evidence can be used if the contract is silent on a particular issue.
That said, if you assert that the timeframe agreed was 2-3 days, you have the onus of proving it if it is denied. Assuming that you and the contractor are the only witnesses to the agreement and that you are equally credible witnesses then you will fail in your burden. Remember, that just because you recall an agreement for the work to be done in 2-3 days doesn't mean it happened or that it happened in exactly the way you remember - human memory is fallible. Is it possible that when you heard that the work would be done 2-3 days after the quote what was said that it would be 2-3 days after work started? These are very different things.
Notwithstanding, this is only relevant as one of the circumstances surrounding whether the delay is reasonable or not.
What would hold up in the court of law?
As discussed above the critical issue is - is the delay reasonable or not?
If the court finds that the delay is reasonable then the contractor has not breached the contract and you lose.
Even if the court finds that the delay is not reasonable, you are only entitled to terminate the contract if the delay amounts to a breach of a condition of the contract. Terms of a contract are either conditions or warranties - breach of either entitles the wronged party to sue for damages but only the breach of a condition entitles the wronged party to terminate the contract. A warranty can be elevated to a condition if the breach is so egregious that monetary damages are insufficient restitution.
Terms relating to time are warranties unless they are explicitly conditions (which is what the phrase "time is of the essence" means).
So, even if a month's delay is unreasonable it is probably not so egregious that your only alternative is to terminate the contract. As such, you are probably in breach by repudiating (terminating without cause) the contract and you will still lose.
If you lose, you will be required to restore the contractor to the position they would be in if the contract had been completed. They are entitled to any irrecoverable expenses they have incurred (trust me, if they go to court they'll find some) plus the margin for overhead and profit they would have received for completing the contract - for a small contractor 30-40% of the contract sum would be justifiable. There is no such thing as "no harm, no foul" in contract law - he expected to profit from the contract and is entitled to do so.
Time can be made a condition subsequently by giving the contractor a reasonable time to finish and putting them on notice that you will terminate the contract if they don't. Its key that this is reasonable - the contractor agreeing to it would make it so but, in the absence of agreement, a generous time is more likely to be reasonable than a miserly one.