HOW LONG IS COMING SOON ?

Any "coming soon" that is over 30 days defeats the purpose, in my opinion. Those in the neighborhood will start ignoring it as will any prospective buyers. The flip side, is the listing agent may be trying to use this as a "pocket listing" to drive all inquiries directly to them off the sign and not the MLS - a practice I also have an opinion on. For myself, I will typically only place a "coming soon" sign if it is a few days before listing as the seller's make the final preparation and we are waiting on photos. Consumers are not going to wait over 30 days "in case" it the perfect home for them. Most are in a time constraint and aren't going to wait around

Within a few days, allowing the seller to make the property ready for show.

The, "COMING SOON" sign offers a bit of flexibility for both the Agent and the Client. At present, I have a Client which insisted I place her property on the MLS. However, there are two issues.
1) My client is in the process of moving out. If you have ever shown an occupied, in which the Occupants, are in the process of moving, you may understand why I attempt to avoid this situation. Properties don't present well with boxes strewn throughout the home. Prospects can't concentrate on the property with boxes and trinkets everywhere. Further, It simply strikes me as unprofessional to present an untidy property.
2) In the event of minor repairs, it may prove beneficial to show the property prior to repair completion. The priority should be Prospect safety.
3) If you have an unique property which is not quite ready for a multitude of reasons, the, "COMING SOON" sign may result in building interest in the property. At best, it may attract a Buyer prior to an actual listing date coming to fruition.
The flexibility comes in when a Client strongly insists upon listing the property prior to it being completely ready. The, "COMING SOON" presents a very reasonable compromise. The downside is you never really know exactly when the property will be completely ready. A one-week time horizon can easily balloon into three-weeks.
I suggest using the, "COMING SOON" sign very sparingly.

I think personally if the agent has an agreement but the seller is not quite ready, maybe doing a few repairs, coming soon for no more than 30 days is reasonable. Anything other than that I would be skeptical if that is a tactic. I recently had a client that asked me to do that because they were getting ready to put the house on the market and the flood came. So since the flood she said she wanted to test the market for a month to see if she gets any interest since her property did not flood.

Answer is a reasonable amount of time. The "coming soon" signs don't have quite the impact in this balanced buyer leaning market. I think now they can be used as a "notice," nothing more.