Depends on the tubes you use...
I would say that if you use NOS tubes (especially the expensive and rarer ones), apart from 1/2 to 1 hours for warm-up, you should avoid leaving it on for longer than neccessary. Since I understand that NOS tubes were constructed better than the majority of new tubes, I would think that they would be able to handle the major shock of being turned on and off, as mentioned by Hirsch, much better than new tubes.
On the other hand, if you are using new tubes (i.e. non-NOS which are realtively cheaper and more readily available), I see no harm in leaving them on, although you must be prepared for a much shorter lifespan.