DHC6 FLT CREW DURING A VFR FLT ENCOUNTERS WX; CLBS TO AVOID IMC AND EXCEEDS ALT FOR USE OF CREW OXYGEN.

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WHILE ENRTE FROM NORTH LAS VEGAS; NV; WE ENCOUNTERED INCREASING CLOUD LAYERS. WE CLBED FROM 11500 FT TO 13500 FT MSL TO TRANSIT OVER 1 CUMULUS LAYER I THOUGHT WAS A NARROW BAND. IT WAS NOT SKINNY; THOUGH; AND IT BUILT QUITE RAPIDLY. I INSTRUCTED MY VERY GREEN SIC TO CONTINUE THE CLB TO KEEP US VFR. I CLBED US TO 15000 FT OR A BIT MORE AS WE MANEUVERED TO REMAIN VFR. I KEPT THINKING IT WOULD BE FOR 'JUST ANOTHER MIN' AS VISUALLY I THOUGHT I SAW VFR BREAKS. HOWEVER; THE VERT DEVELOPMENT WAS GREATER THAN I HAD JUDGED. I HAD A MOMENTARY HESITATION DUE TO MY COPLT'S INEXPERIENCE WITH ATC; BUT I TOLD HER TO TELL ATC WE NEEDED AN IFR DSCNT (TO GET BACK TO THE CORRECT ALT). THEY TOLD US TO STAND BY AND THEY WOULD WORK AT GETTING A CLRNC FOR US. IN SUMMARY; I ESTIMATE I WAS ABOVE THE 'REQUIRED OXYGEN FOR CREW MEMBERS' LEVEL FOR APPROX 10 MINS. WE SAW A LARGE VFR CORRIDOR AND MADE A VFR DSCNT BEFORE CTR COULD RETURN WITH A CLRNC. IT WAS NOT A GOOD JUDGEMENT CALL TO JUDGE A CUMULOUS LAYER ON ITS INITIAL APPEARANCE. I DO A LOT OF LOW LEVEL FLYING IN MY DIFFERENT JOBS; AND A LITTLE VOICE TOLD ME NOT TO TRY TO HOP OVER THIS INNOCUOUS-LOOKING LAYER. I WILL LISTEN TO THAT VOICE BETTER NOW.