Disclosure - I run an Upvote panel, just so you're aware and don't think of this as bias-laden answers or me promoting something.
1) You wrote a comment today, how many hours/days are going to wait until buying upvotes for your post/comment?
Recency is a very prominent metric in Reddit.
But I can also tell you with almost certainty that the rules for Reddit are extremely different for different subreddits. We've seen that they can be a certain way for prominent subreddits, while acting completely differently for up-and-coming subs and a third layer for relatively new subs.
That being said, this is definitely not the only metric. We've found that there are certain subs with a high likelihood of abuse that operate under a constant flag for self-promotion (over and above hyper-enthusiastic mods). Country specific subs also seem to behave different.
So this becomes a game of trial and error with your target subs. Much like ads.
You should typically send a small bunch of deferred Upvotes (slowest time batch at your provider) within minutes of you make the post, so it starts garnering attention.
Then gradually keep placing larger and larger orders as time goes by to maintain top comment position.
2) When you buy upvotes, you prefer to select dripfeed or not? Receiving an exact amount of upvotes every hour can be a detectable pattern? Or is it better to just buy tens of upvotes at once?
This depends upon the competition.
In hyper competitive niches it becomes a game of numbers. Since even if you were the first, someone will soon come along using Reddit listener systems and post a competing comment.
If your top competitor is sitting with 2.8k upvotes, you have to find a way to beat them.
If you're already there with fewer upvotes, your job is mostly done. All you need to do is constantly monitor the post to ensure you're the top comment.
Deferring post also is highly provider specific. Some use rapid programmatic pushes that will send all your votes in a short amount of time or x votes per hour delivered without deference. Others will have internal heuristics in place to ensure even if you set it to certain time sets, the pattern remains random and ambiguous.
I don't have answer for your third question unfortunately as I have no experience with that.