There is some confusion for me here, as steep can mean the tobacco steeping in the solvent, or, after the tobacco is removed and you just have a tobacco extract, the tobacco extract can be steeping with added nic/pg/vg. So there are 2 steeps. The first one finishes when the tobacco plant material is seperated from the solvent. The second one begins when nic/pg/vg is added and it will usually be pretty rough for the first couple of weeks, reasonable after a month, and even better after 2 months or more IME.
Ian,
Actually, there are three steeps, each of which affects the amount and balance of flavors in the profile:
Steep #1---during the maceration
Steep #2---after the extract is filtered and bottled
Steep #3---after the juice is mixed
Obviously, the first steep is critical, going from an unflavored solvent to a flavored extract. Much of the essential character of the extract is determined by the maceration process, where the flavor components of the tobacco gradually seep into the solvent. How long the tobacco should steep in the maceration is affected by numerous variables: type of tobacco or blend (single-leaf, cigarette/RYO, pipe blend, or cigar), freshness, moisture content, cut, type of solvent or solvents used, temperature of the maceration (room-temperature or heat-assisted), etc.
The second steep, however, is also very important. Letting the filtered extract "age" in the bottle alters its flavor profile, sometimes dramatically. Extracts made from simple-soak macerations start out emphasizing flavors other than the tobaccos themselves, especially sweet flavors applied to the tobacco. These other flavors come in three categories: first, as water-based sprayed-on "casings" (that seep into the tobacco) which can be applied to the growing leaves before harvest, after harvest, or during processing/curing; second, alcohol-based dipped "toppings" (that remain on the surface of the tobacco), applied either during or after curing; and third, additional flavorings added during the final blending of the tobacco.
Virtually all retail tobacco is cased---not necessarily as flavoring, but to minimize unwanted characteristics of the tobacco when smoked, such as harshness or excess bite. Some tobaccos are then topped with other flavors. When the extract is freshly filtered, these other flavors are at their strongest. Over time, however, sweeter flavors diminish in the extracts and the pure tobacco flavor tends to strengthen and move forward in the flavor profile.
That same process occurs in the mixed juices with the third steep, especially in complex mixes that add others flavorings. Typically, the third steep produces the least dramatic results when the juice is mixed only with the natural tobacco extract, but somewhat more obvious when the mixed juice is a "hybrid" of the tobacco extract and other natural or synthetic non-tobacco flavorings.