Short term memory is conscious memory since we can draw upon it at will, though it is fleeting memory. Semiconscious refers to a physiological state, not a cognitive process, in which a person is semi-aware of sensations and only partially able to respond to stimulus as when half asleep or in a medical emergency; this is a physiological or medical condition, not a cognitive process related to memory (in fact, when one is semiconscious due to emergency, one's short term memory may convert under the impetus of strong emotion to conscious long term memory more quickly).
There may be some disagreement between studies, but the accepted average duration for short term memory, according to Atkinson and Shiffrin (1971), is between 15 and 30 seconds, though some types of short term memory, i.e., auditory, are theorized as being perhaps as little as 1 to 2 seconds. In contrast, long term memory may last between minutes (as opposed to seconds) and years or decades. For instance, in a 1975 study by Bahrick et al, 30 percent of participants recalled high school classmates' names 48 years after graduation. Thus the comparison between duration of short and long term memory is contrasting, not comparable.
In a broader view, short term memory theory is unitary (with only a single system of memory storage and no subsystems) and has generally been replaced by the theory of working memory, which theorizes multiple systems of memory storage for multiple types of memory.