However, it only lasts in the soil for 10 to 14 days. Whereas F,B and B is viable for 30 days plus.
The nitrogen may only be available for a short time, especially if there is a lot of rain, but it will usually be longer than 14 days,
The potash and phosphate will be there season long as they are less soluble than nitrogen. Potash and phosphate fertilizers can be applied in the previous Autumn and are still available in the Spring whereas nitrogen is washed away to pollute our rivers.
The same really applies to the nutrients in FB&B although they are released a little slower as the organic materials in it have to be broken down by soil bacteria to release them. Conversly the nutrients are not immediately available to the plants, and sometimes you need them to be.
As stompy says - READ THE INSTRUCTIONS ON THE PACKET.
The usual way to use Growmore is to put it on the soil about 10 days before you are going to plant/sow and work it into the top few inches. If you put it on too near to sowing it can burn the newly germinated seeds. If you put it on too early the nitrogen may be lost.
Top dressing with Growmore is of little value as only the nitrogen is really soluble and mobile in the soil. The potash and phosphate tend to stay in the surface layer and not move down to root level unless they are worked in. Better to top dress with nitrogen or use a liquid feed.