You can club maximum 5 energies related to same healing aspect at a time.
This applies to all heaings including Perfect Health Serum.
VK has the ability to send energy for 30 min by auto healing. So yu can place request for next healing aspect after around 30 min.
When need is more you can send energy every 10 min.
Please do read the posts on Basics of VK section and hyperlinks to understand in detail.