• Both SLC and MLC theoretically have unlimited reading lifespan, but writing lifespan is limited.
• SLC NAND flash has about 30,000 write cycles.
• MLC NAND flash has about 3,000 write cycles.
• Example 1: A MLC SSD is 32GB, and has 10GB empty space; then the estimated writing lifespan is 10GB x 3000 = 30,000GB. Assume an users write date to SSD about 30GB/day; 30,000GB / 30GB = 1,000 days. the same SSD
• Example 2: Intel X25 34nm 80GB, which estimate has 1,000 write life cycle; if it has 10GB empty space; 10GB x 1000 = 10,000; if write 30GB per day, its writing lifespan is about 330 days only.

SLC main advantage over MLC is the writing lifespan and writing performance. You should consider SLC based SSD if:

• You have 1GB RAM or less. because your computer will use HD/SSD as memory, therefore write performance and lifespan is very important.
• You do a lot of video/audio editing, or BT downloading.
• Generally, SLC has approximately 10 time lifespan of MLC.

Normally, system with commonly used software together won't be over 10-15GB for average users.

Solid-Sate Drive is different then regular Hard Drive, it's NOT required for defragmentation. SSD has seek time around 0.1ms only, while the fastest Hard disk nowadays has seek time at least 7 to 10ms.

Defragmentation will reduce lifespan for SSD. Frequently defrag SSD will damage it's NAND flash.