If I've submitted a proposal today then how long it will be able to show for clients? - As long as the job is valid/remains open on Upwork. It doesn't matter if your proposal can be seen after a month or an year. If a client wants to hire you he will do with in 3 or within 7 days(rare case). If he don't want to hire you then your proposal can't force him to hire you.
So the time period in which the proposable is viewable doesn't matter at all.
Is Upwork supports for newbies without upgrading membership? - Only you can support yourself. Memeber Upgrade can't help you to get the job it can only help you to get extra connects or gives you an option to hide your earning ;')
Does this help?