I'm getting to understand something here, manually selecting randomness of seed and seed extension( I mean adding additional word making the 13th word) is the same thing? Will it be possible to crack/brute force extended mnemonic seed phrase, if possible, then how long can it takes to be done.
My questions might seem to be out of context, correct me if I'm wrong, I'm learning a lot.
No they are not the same thing, the seed phrase itself (the 12 to 24 words) must be generated completely at random using a strong RNG and never manually. The extra word that extends this can be selected by the user "manually" because it was not meant to provide security but only "plausible deniability". Keep in mind that the extra word does NOT encrypt your seed, it just extends it using a very weak KDF.
Breaking it depends on the phrase that was used as the 13th "word". For example
123 is easier to brute force than
?Z1y-R?lKT/}. The time it takes depends on the number of characters in that word, type of them (upper/lower case, numbers, symbols) and whether it is actually random or a known phrase, meaning a poem or a famous quote like "remember remember the fifth of November" is not considered safe even though it is long (39 byte).