Your question isnt very clear. It sounds like you asking how big a ReadyBoost cache can be on your SD card.
From Technet:
You must reserve at least 256 MB. Larger caches can improve performance, but the ReadyBoost cache cannot be greater than 4 GB on a FAT32 file system or greater than 32 GB on an NTFS file system.
ReadyBoost creates a disk cache file named ReadyBoost.sfcache in the root of the flash drive. The file is immediately created for the full size of the specified cache. However, Windows will gradually fill the space with cached content.
Even if the ReadyBoost cache is filled, that doesnt mean it will read from it. ReadyBoost will use SuperFetch to predict what data you might read in the future. This doesnt guarantee the data it puts in the cache will even be read.
More than likely, on your machine, you are writing lots of data to the ReadyBoost cache, but not actually reading from it. You can see this using PerfMon and adding the counter Cache Read Bytes/sec.
I strongly suggest you read that Technet article, as well as this article. You are getting very little, if any benefit from ReadyBoost on your 8 GB machine.