Originally Posted by
quartz My book says
пора + imperfect infinitive = "it is time to"
пора + perfect infinitive = "it is necessary to"
It may be a good rule in a sense that if you use it you will be much more often right than wrong, but, concerning the second part, native speakers are often quite happy with both kinds of verbs:
(Looking in the mirror) Мне пора (по)бриться.
У меня на компьютере совсем места не осталось. Пора диск (по)чистить.
На даче мокрое пятно на потолке. Пора крышу чинить. (this one is against the rule as far as I'm concerned)
На даче опять мокрое пятно на потолке. Пора крышу починить наконец. (this one confirms to the rule strictly)
(Looking in the empty fridge) Пора идти/сходить в магазин.
Of course, one might say that "Мне пора (по)бриться" is of the "it is time to" kind and at the same time of the "it is necessary to" kind. If this is the case, I will be only too happy.