A question akin to "how long is a piece of string?" I hope knowledgeable readers will forgive me rehearsing the "received wisdom". Malting is the process of causing the barley seed to sprout and in the process release/produce enzymes (basically "amylase" and perhaps "protease") that inter alia can be used to convert contained starch to fermentable sugars. So to a large extent the efficiency of the malting process can be judged by the diastatic power of the resulting dried malt. The thing that sets malt apart from roast grain and unmalted grain is that diastatic power - the ability to convert starch in the grain into fermantable sugar when mashed in solution.
Given all that the question can be put in context. How to make the grain sprout, keep it fresh long enough to let the biochemistry work and then dry it quickly as possible to preserve all the good work. Mouldy malt is not very useful.
The question concerns steeping the grain in water. The grain is usually submerged in water for some time. It seems not to really matter as long as the period is no longer than (say) 12 hours. But that is temperature dependant - cooler allows for slightly longer and heat would shorten the optimal time. The problem, it seems, being spoilage (fermentation?) of the grain by wild yeasts (etc.) in solution. So draining the grain before that can happen is the most limiting factor for soaking time. There is some argument that submerging the grain in water for too long might cause it to "over hydrates" making it split and become "mushy". That would require more than a day so 8 hours would seem to be a good upper limit for any initial soaking.
In practice the grain needs to be soaked to efficiently hydrate the dried seed kernel and initiate germination. That is most efficiently done under water but it can be done by continuous spraying/draining for a similar time. Once initiated the germination process requires oxygen. So keeping the grain underwater would inhibit that requirement. However the germination process also requires water and over a few days the grain is likely to dry out and inhibit further necessary biological processes. So a close observation must be kept on the malting grain to make sure it does not dry out before it has correctly sprouted. How quickly grain can dry out is a local, usually meteorological, factor and not every prescribed malting method fits every situation. If one makes malt in a particular location many times then a "process pattern" starts to emerge. In practice one can start with any recommended plan and see how it goes. Just keep an eye on the grain for sprouting (good) and moulding (bad).
As a recommendation, I would advise steeping fresh grain for less time than long stored (old) grain. Fresh grain would probably only require 2 hours of so of soaking. Older dried grain may well benefit for a longer soak. Other brewers I know here in UK have left the grain soaking "overnight" and have not reported a problem. Most brewers (amateur maltsters?) I have talked to do a second soak a day or two after the first soak - but only for a hour or less. Some soak for an hour three times in 3 days. That makes sure the grain is fully hydrated (but not water logged) and can deter/wash off any surface moulds that may be developing. After two/three days of wetting the grain they begin to air dry the malted grain. After about 5 days being turned on an open tray/boarded area, the grain is fully dried(kilned?) in a very low temperature oven.
The diastatic power of home made malt can vary widely and is usually only optimal or repeatable after a number of runs using variously sourced grains in varying conditions. It also requires a reasonable amount of space (not the kitchen table!) for a week. It can be a "fiddly process" getting it right but great fun once cracked and a viable/optimal localised procedure has been confirmed. Good luck!