What is standard Reformed Baptist practice regarding both the timeline of baptism for either new believers or new members who were baptized as infants?
New believers: you fill out a membership packet that outlines your testimony and you need to speak to a pastor/elder you are under/close to. Once they sign off on it, you write your testimony out as a speech and present it to the membership office that will check it for gospel clarity and appropriateness (e.g. being unnecessarily specific regarding sin). Then the new believer is put in a queue to be baptized on a Sunday evening service where they will read out their testimony they had previously submitted to the church and then be baptized then and there. Some time after that, there will be a "Right Hand of Fellowship" presentation on a Sunday evening when all the new members are presented to the church and current members are welcomed, after the service, to come up and welcome them into the local congregation. New members who were baptized as infants: if the new believer is convinced their infant baptism was legitimate, they will not force you to get rebaptized (we are not Anabaptists! haha). Therefore, they can skip the whole "baptism and reading their testimony to everyone" part. But if you think your baptism in a true church was not legitimate, then you can choose to be rebaptized (are we Anabaptists now? xD ) Also what is standard practice regarding who is allowed to participate in the Lord's Supper?
That is left to everyone's conscience. The pastor will tell the congregation who can take it (believers) and how to take it properly but there is no physical "fencing of the Table". Are those wishing baptism usually baptized within weeks, months, years?
Usually within a few months Are they allowed or denied the Lord's table until that point?
See above (left up to the individual's conscience). Guests who are professing believers take it and the church seems to not have an issue with that either. What if they were baptized as infants in a true church?
See above again Thanks.