How to use 'since' and 'for' in present perfect tense

Two words which are good indicators of present perfect tense are 'since' and 'for'. To use these two correctly with the above tense, note the following rules:
1 'for' is followed by a duration of time. e.g. for three hours.
2 'since' is followed by a fixed point in time. e.g. since last week, since 8.00 a.m.

Let us look at the following sentences to see how 'for' and 'since' are used in the present perfect tense:
(i) I have waited for him for two hours.
(ii) He has been absent from school since last Wednesday