Mostly it's up to you and your company's policy. Also consider the more restore points you keep, the more storage is used.
Generally, the default value in B&R is set to 14.
Also, it depends on what kind of VM you are backing up - as Brian mentioned in the previous post. You may also take a look at this post of Tom (for Exchange): Re: Veeam Exchange best practice, this post of Vitaly (for tapes, bullet 3): Re: Veeam 4.1 Job Scheduling Advice or just use forum search for more info.