It is great if someone could help me about The permitted time for operation the Steam turbines at Full speed no load condition?
You're going to be surprised by this, but the allowable full speed-no load operating time is different for just about every steam turbine. The manufacturer decides what is allowable and what is not, and in some cases the actual installation also affects that ability or time limitation. And, some steam turbines can't operate for any appreciable length of time at no or low loads, while others can with no time limits. Some steam turbines have exhaust hood sprays to help protect the last stage(s) of the turbine at no- or low load operation, and some don't. Some steam turbines experience condensation before the condenser at no-or low loads, which is usually quite bad for blades and nozzles and packing, while others can withstand mid-turbine steam condensation.
Really, you should be asking this question of the supplier and/or manufacturer of the steam turbine in question. Or, you need to provide a LOT more information in this post about the steam turbine in question.
Hope this helps!