MAVS NEWS:

[F Gump]

Is there a date you need convert a two way?   With how the season is going, I will be disappointed if they don't find a way to convert at least one on a rest of the year plus a non guaranteed minimum next year.   Maybe use the next few weeks to see which veteran you want to cut loose.  Ideally giving them the opportunity to sign with a playoff team.  For me it is either Jones or Middleton.   Maybe one can stick next year, but I don't want both back.

It would be a negotiation (since it's a new contract). That has some factors in play which you didn't ask, but which factor into what might happen. IMO the path forward is NOT obvious. 

1 The earlier you do a conversion after the 2-way NBA eligibility (which is NBA level pay) has run out, the more extra money the player gets. 
2 If you don't convert the deal at all, then he stays your GL player after his NBA eligibility ends, for the rest of the season. 
3 If you have him in GL while the new guys play, he is not using up any NBA eligibility. And he gets more minutes to work on his game. 
4 The conversion can be to ANY contract you can legally offer, so it can be up to 4 seasons (remainder of this, plus 3 more, if you have an exception that allows that). IIRC the Mavs have no exception other than minimum which is 2 years.
5 The player wants an NBA contract and it would be unlikely they would say no to anything offered. But not impossible -- he might prefer to test free agency in the summer.
6 Mavs have no real need to convert anyone. There's no benefit to this team UNLESS they are getting a future year. 
7 Mavs might prefer to have the player back as a 2 way next year, which is only possible if he is NOT converted to an NBA deal for next season.
8 When the player becomes a free agent, he will be RFA. 

Back to the question of the last day possible for a conversion, it would be the last day a contract can be offered, so it would be prior to the last game of the season. But all the factors above tell me that would not be the plan. 

FWIW, last season BWill got converted with about 5 games left, and another season. But that was hampered by financial restraints that aren't likely to be in play this season. 

They do have a need to stay under the tax line, but iirc they have enough cushion to comfortably do 2 conversions with room to spare.