let me share some of my understanding here, i stand to be
corrected by the seniors.
dondontan, to address your queries
liberation is to get out of endless cycle of birth-and-death,
meaning you are no longer at the mercy of your own karma to undergo
rebirth over and over again. The main reason why this is even
possible is that the mechanism of birth-and-death is maintained
based on the illusion of a self. This illusion does not
conform to reality and when it is dissolved through meditation on
emptiness, then one can stop taking rebirth, driven by
karma.
There are however two main approaches to doing this. This
is usually called the Hinayana and Mahayana paths. The
ultimate goal of the Hinayana path is to attain the fruition of
Arhat. There are generally four categories of fruition on the
Hinayana path all labelled enlightenment but it is only at
Arhatship, the fourth level of enlightenment that one is
actually liberated from birth and death cycle, the prior three
has not completely actualised the wisdom that realises selflessness
completely.
On the Mahayana path, it is said that the fruition is
Buddhahood. Which is the ultimate conclusion to the spiritual
path because he has perfected both the benefit for himself and the
benefit for others. To achieve that, one has to perfect both
the stores of merit and wisdom. And the practitioners on this path
has to take the Bodhisattva vow, generate the intention to liberate
all mother sentient beings (the intention of bodhichitta, or
awakening mind) and also practice the 6 or 10
paramitas...
In terms of time, it is said that the Arhatship can be achieved
in three lives. While the Buddhahood takes three countless aeons in
the general mahayana path and can take up to 16 lives, 7 lives, 3
lives or 1 life in the extraordinary path of vajrayana which
is a subsect of the mahayana path, utilising special skilful
means.
There are many differences between Arhats and Buddhas but
i'd outline just a few that i can think of. Firstly
Arhats while having being liberated from samsara, have not reached
spiritual perfection, this is because while they have realised
fully the emptiness of self, they have not actualised fully
emptiness of other phenomenon. In terms of obscurations, they have
not fully purified the obscurations to omniscience leading them to
be still far from the state of full omniscience (all knowing) that
the Buddhas possess. For instance, in terms of their knowledge of
past lives, it is said that Arhats can only see back into 500
lives, while Buddhas can see the three times (past, present,
future) unimpededly without limit.
Furthermore, it is said that the Arhats due to not having
emphasized the first 5 paramitas and accumulated merit, nor
generated Bodhichitta, being more or less focussed on only
liberating himself from samsara only, is not able to accomplish the
extensive benefit of sentient beings as a Buddha is able to through
the limitless emanations of his form-bodies (rupakaya). At a
certain point, after a very long period of time in the state of
nirvana (state of extinction/cessation), the Arhat will be awakened
from his samadhi (deep meditation) by light-rays from a Buddha
and exhorted to engage in the Mahayana path and thereby go on to
traverse the Bodhisattva stages, and in the end, accomplishing
Buddhahood too. Thus it is said that Arhat path is a
prolonged path, from the perspective of attaining Buddhahood.
Of course, this is the general explanation, in practice, there
are always exceptions and special circumstances.
If we are talking about Bodhisttvas like Guan Yin, Di Zang Pusa,
etc. They have vowed to put aside their own Buddhahood and focus on
working to save beings and bringing them to Buddhahood. Strangely,
it seems that this altruism is the very source of the high
spiritual qualities they possess and it seems although they have
vowed to go behind everyone else, but they are by some natural law,
already far ahead... anyway i digress here.
If one is talking about ordinary people attaining the fruitions,
i assume that you are talking about people with no prior contact to
Buddhism in past lives, having only started on the path in this
life. In each case, it is hard to say because everyone's
store of merit is different. However, whether one is in the
category of ordinary people or not is also not sure. Maybe
one has already been on this path before, only one does not
know.
The Buddha kindly bestowed countless methods to liberate beings
and each method suits different people. Similarly, walking
either the Hinayana or Mahayana path is dependent on one's
inclinations and past karmic connection.