Steve,
If just one bunker is being rebuilt it shouldn't take any longer than say a couple days, but this can depend on many factors which are also applicable to any bunker project, small or big, oh yeah, SIZE does matter!.. on weather, area of disturbance, if irrigation is being added, if you are sodding the surrounds, the style/type of bunker design, if you have a skilled contractor, etc.
An average bunker size is really relative to the architect doing the work or the size/type/style of the original architect's work. For discussion sake, let's consider bunkers around 800 SF, a decent bunker for many older courses, but perhaps considered small by some of the more modern designs.
What do you consider a "long time" to be? When did "they" start how many bunkers are they completing, what has been the weather conditions during construction?
Costs: this too is an open ended statement and can fluctuate considerably based on, once again, size, style, type, chartater of design, materials used such as sodding all the surrounds, the sand, amount of internal and external drainage installed, location on the course, is it difficult to get to...is soil required or does it need to be removed to achieve design intent? and so on...
Grass faced bunkers: how steep are the faces, will additional irrigation be installed, is there much cut involved, does much old sand need to be removed?. You also have to remember and give consideration to construction techniques required by the architect or used by the contractor. Some will cost more, but they may not necessarilly result in better bunkers.
For the most part, yes there are too many variables involved with your initial premise to give a decent answer regarding costs, but here is an example to consider. We are in the middle of a large scale bunker reconstruction project where the contractor is completing 73 bunkers ranging from several small intimate bunkers at 300 SF to a few larger ones at 2,000 SF on an old course. The Owner is buying all the construction materials and doing all irrigation adjustments needed and the contractor is providing the equipment and labor to perform the essential of the work. The total costs which included our fees was between $390,000 and $430,000.
Naturally you can do the math, but some bunkers will of course be quite reasonable at say $3,500 each and others will not top off until they reach $8-9,000