You are going to get many opinions on this, and heres mine.
You should shut down at night.
If you use it many times during the day leave it in sleep or hibernate mode.
A computer should be shut down at least once a week so minor problems can be corrected during boot.
Computer type PC/Desktop OS Windows® 7 Home Premium SP-1 CPU Intel Pentium(R) E5200 @ 2.50GHz Motherboard Intel Corporation DG41WV (PROCESSOR) Memory 4096 MBytes Dual-Channel DDR3 @ 398MHz Graphics Card ATI Radeon HD 5450 (ASUStek Computer Inc) Sound Card Realtek High Definition Audio Monitor(s) Displays DELL E170S on ATI Radeon HD 5450 Screen Resolution 1280x1024 pixels
Keyboard Logitech PS/2 Keyboard Mouse A4 Tech Co Ltd/Logitech PSU 400W Case Nothing Fancy Cooling Fans Hard Drives 466GB Western Digital WDC Internet Speed 512 Kbps Antivirus Microsoft Security Essentials & Malwarebytes Browser Chrome & Internet Explorer 9 Other Info Imation 8 GB USB, 6 GB Seagate External Hard drive, ASUS DVD-E818A7T ATA Device
Well, I often just let my laptop run for days on end. Maybe, as said by richc46, yes, maybe once a week. But, as so often with Windows, there is always a update around the corner, that want's you to restart
System Manufacturer/Model Number Lenovo W510 OS 64 bit Windows 7 CPU Intel Core i7 Memory 6GB Graphics Card NVIDIA Quadro FX 880M with 1GB DDR3 Monitor(s) Displays 15,6" + 21" NONAME (neovo) Screen Resolution 1920x1080 and 1440x900
Keyboard Microsoft Ergonomic 4000 (got two of them :) ) Mouse Logitec Darkfield Hard Drives Laptop: 300GB (seagate? Have not checked yet)
External: 1x Lacie 320GB, 1x Enclosuer w. 160GB Internet Speed 20Mbit/ 2Mbit
There is no time or frequency that's good or bad. You can get into issues with the extremes at either end. There's no reason to do it every few hours, unless you need to apply an update, driver update, etc. You also don't want to shut it down and power it up too often. However, there's little to no reason to keep it running 24-7, either. I don't use my computer while I'm sleeping, so there's no reason to let it run. I never understood people's reluctance to boot up a computer and would choose to leave it running all the time.