(Disclaimer: this represents my own opinions, and has only been written as "fact" because it's annoying to add appropriate "in my opinion" or "my interpretation is" statements to every sentence. So please take the following with the appropriate grain of salt; your mileage may vary.)
A quick ritual is up to somewhere around lore +2 or so; you can burn a fate point or accept a minor consequence to push a bit beyond your normal limits without having to slow down too much. This requires a minimum of a minute to set up (call that maybe six to ten exchanges if there's physical combat going on at the time, during which time anyone can disrupt things), followed by however many exchanges are needed to call up and control the necessary amount of power. Most uses of ritual magic should fall into this category.
A normal ritual tops out at around lore +8; if you're running purely off of fate points, consequences, and declarations, you can probably get such a thing set up in five to ten minutes; if you're using maneuvers, you need at least a full scene for each. Under normal circumstances, when you're not in any kind of rush, this will take two or three hours to set up.
A powerful ritual runs up to around lore +14; for a practicing blood mage this can still be fairly rapid, requiring only a full scene (maybe 15 minutes?) of preparation before casting can commence. More normally a ritual of this sort would require a day or three of preparation, including at least one scene with more than a simple conflict to it; whether that's negotiating with Bob for assistance, or fighting off angry corvids to get an unhatched raven egg, or tracking down and purchasing a fireplace poker that's been in use for at least 100 years... Regardless, unless you're taking serious shortcuts (i.e. blood magic), this isn't the sort of ritual where you can just roll the dice for a bunch of maneuvers and declare it done.
Anything beyond that level is an extreme ritual and should require a full adventure to just get the preparation done; even for a blood mage, operating at this level of power requires either a fairly significant cult following or an actual sacrifice (and corresponding first law issues), either of which come with their own problems.
The possession of an appropriate sponsored magic may, in some cases, allow some reductions in preparation or casting time, though whether any given spell gets such benefits is up to GM whim.
I'd also note that just drawing a magic circle is a lore-based maneuver, not a ritual, and can thus be done in combat without too much trouble.