How it is used?

Depends on what the customer needs. That depends, as you know, since the FPGA are programmed by the customer when they receive it (hence the field programmable part) then it depends on to what extent the customer will be programming. Some FPGAs are programmable for very simple routing like AND/OR, and some perform very complex routings. Also it depends on how much memory the FPGA contains, the more memory the higher the cost but this usually does not mean longer time to the field.

Time to market and cost?

Most FPGAs are ordered for a specific need the customer has, so the manufacturer usually has a very good idea about how long it takes to get it (them) to the customer.

i know all this but if fpga compare with the other pld or logic devices ?i mean the time to market and the cost is it will much different?

I have used Altera and you can take you CAD electrical schematic and download it into the FPGA or use a boot ROM and you have your product. The advantage you can reprogram it any time you choose.