I'd talk to your builder... there are lot's of different types of linseed products, if you used natural linseed oil you are looking at weeks of drying time, you could have added solvents such as mineral spirits, japan drier...
Still I'd have a talk with him/her the idea of having linseed oil on the mantle is a bit scary, he'll be able to tell you if he's exceeded the limits of the mantle placement, and projection which depend on the gas fireplace model and what they recomend so it won't catch fire..
...you have possibly made a mistake using linseed here.
Below from: http://www.naturalhandyman.com/iip/infxtra/inflin.shtm
Let me quote the warning label from a can of Ace Hardware brand boiled linseed oil...
"Use of this product will expose you to arsenic, beryllium, chromium, cadmium and nickel, which are known to cause cancer; and lead which is known to cause birth defects and other reproductive harm."
OK, so that doesn't scare you. What about spontaneous combustion? You know, like in the Sci-Fi movies when people suddenly burst into flames.
When linseed oil dries, it releases heat. The more linseed oil, the greater the heat. A pile of rags or paper towels soaked with linseed oil can actually start burning without warning, leading to the manufacturer's warning that all oil-soaked rags should be stored under water in a covered, metal container.
By the time you add a mildewcide... well, you get the picture. So much for linseed oil being environmentally friendly. Face it... sometimes the most environmentally unfriendly person around is Mother Nature.
(the above refers to using it with rags but you can see it is highly combustable)
Some of the problems with straight linseed oil, boiled or raw, are:
No UV (ultraviolet) light resistance... UV causes more damage to exposed wood than any other factor, destroying wood fibers and setting it up for attack by mildew, fungus, and insects.
Linseed oil is mildew food... Many vegetable oils are food products for humans... all vegetable oils are food products for mildew! Linseed oil is not completely denatured, so it can encourage rather than discourage mildew growth.
Linseed oil does not harden sufficiently to offer enough resistance to abrasion to be a suitable deck floor preservative... at least by today's standards. Linseed oil has been used for interior wood floors, but it must be waxed for durability!
Difficult to remove from wood... Multiple coats of linseed oil are gummy and difficult to remove fully for refinishing
Alter Eagle Construction & Design