I heard from one of my friends that half of current Brahma's time is passed, is that true?
How much time has been passed of current Brahma?
Yes, that's true as per Chapter 46 of Markandeya Purana and Chapter 5 of Kurma Purana.
As per Chapter 46 of Markandeya Purana,
For a hundred of his (Brahma's) years is denominated a Para; and a Parardha or half a Para is well-known to be composed of fifty years.
So then a Parardha of his life has elapsed, O brahman; at the close of which occurred the Maha-kalpa, which is famed as the Padma.
Of the second Parardha which is now passing, O brahman, the first kalpa (or cycle) ordained is this one called the Varaha.
Sanskrit verses and Hindi translation of Chapter 46 of Markandeya Purana can be found here.
Lord Brahma's life-span is mentioned hundred (100) years (in which day is known as Kalpa). Lord Brahma's 100 years are known as Para which is his life span.
And Parardha or half a Para is of fifty (50) such years. In another words, Lord Brahma's life-span is of 2 Parardhas.
And at the end of Padma Kalpa (previous Kalpa), first Parardha got over i.e. half life span of Lord Brahma got completed.
Yes we are in the 2nd part of Lord Brahma's lifetime as evidenced by the well known sankalpam mantra
'adya brahmanah dvitiye parardhe shveta varaha kalpe '.