It's up to you how to track the total XP and decide when to level up (subtract the amount needed to level, leave the running total, etc.). As there are different levelling speeds (slow, standard and fast - page 509) it's not always 1,000xp for every campaign. Keep in mind that all of this is just for manual tracking - FG doesn't do anything when a PC hits a new level milestone, it's up to the player to realise and manually level (drag the class entry to the PC sheet or the chargen tracker). Most players that I know keep a sharp eye on their XP and it's rare for them to not realise when they have enough to level!