It really depends on how much free space you have available on your HDD that you would like to use for this, and how many restore points you would like to be able to keep before the oldest ones get deleted automatically to make room for a new restore point. The average restore point could be up to around 1.3 GB or so in size.
With the above in mind, you'll just need to factor in how many restore points you create, how often you create them, and how many you would like to keep on hand in case you need to use one that is old enough to undo the current mistake made.
If I need to use System Restore, how far back in time will I likely have to go?
Personally, I've never needed to go back more than a week. And I use System Restore at least once a month.
In fact, I usually restore to no more than a day back in time. The problem I am trying to resolve typically has shown itself in the very recent past as a result of something specific that I did.
Some advocate keeping several months worth of restore points. I don't argue with them. I guess if hard drive space is available, there is no harm done.
My system is currently set to use about 7 GB for restore points. Right now, I have 11 restore points dating back to December 6--plenty long enough in my experience.
My Computer
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Ignatz Special; 4 speed manual gearbox; factory air conditioning; one of one
OS
Windows 7 Home Premium SP1, 64-bit
CPU
Intel Skylake i5-6600K, not overclocked
Motherboard
AsRock Z170M Extreme 4, micro ATX
Memory
8 GB HyperX DDR4-2666 (2 x 4 GB)
Graphics Card(s)
none; graphics are integrated on CPU
Sound Card
onboard: Realtek ALC1150; external: USB Behringer UF0-202
Monitor(s) Displays
Dell S2340M 23 inch IPS
Screen Resolution
1600 x 900
Hard Drives
System: Crucial MX100 series SSD, 128 GB;
Data: Samsung Spinpoint 103SJ, 1 TB;
Backup: WD Caviar Green WD30EZRX-00D8PB0, 3 TB
Microsoft Security Essentials and Malwarebytes Premium
Browser
Pale Moon
Other Info
All fans PWM; speeds at idle: CPU circa 500 rpm; intake circa 600 rpm; exhaust circa 600 rpm; CPU temps 27 idle and 47 C load in a warm room (27 C/81 F) when running Intel Extreme Tuning Utility stress test.
I currently have 7.5GB set aside for system restore. Its all a matter of your personal needs. With your hard drive size I would use 5-8%, but its totally up to you to decide for yourself.