srd-05vdc-sl-c and coil operating time I'm using Arduino UNO to control an imatic relay board with 16 relays model srd-05vdc-sl-c.
I use all the relays to control some linear actuators and everything is working fine. 

My only question is: are the coils used for these relays for continuous use? I mean, can I continue to give power to some coils for more than 30 minutes? Or will they be destroyed? 
Should I write a specific Arduino routine to power off all the coils when not used, or can I leave them powered for 30 or more minutes?

I checked the relays and they do not overheat.
You can leave them energized for more than 30 minutes. My 16 year old car with 250,000 km still has the original fuel pump relay. I can't imagine how many hours of continuous use it received over this period of time. The relay in my car is not the same make / model as the relays you are using, so you should check with the manufacture to see if there is a "service lifetime" specification.