how many times can we reuse/write to DVD+RW?

[j p]

am trying to use an Imation 4x DVD+RW media for daily backups in my
Hitachi 4xDVDRW
drive (Dell 8600 laptop)

Searched the growisofs site, but unclear about, as to how many times we can use
a DVD+RW media before it goes bad (if at all it does)..

am using a command something like

growisofs -dvd-compat -Z /dev/dvd -R -J /backupdir/
every time I do this, growisofs indicates it is formating the drive
(take a minute or so) and
then writes a new session..

if the above command is not a good way to use a DVD+RW media, appreciate any 
other ideas.. also any interesting/useful info from folks who use
DVD+RW as backup media??

TIA