slow, steady increments. Faster if you don't take losses and inflict some.
About initial XP. You Sir are for a free ride in data analysis. You asked for it, you'll be well served, NOW!

Check
\VGN\GameData\FacAttribs\23-fat_MilProfessionalArmy.att
UID = 23
Alias = fat_MilProfessionalArmy
Name = $fat_NAM_MilProfessionalArmy
Text = $fat_TXT_MilProfessionalArmy
Kind = $fatMilitary
SubType = $fatMilProfessionalArmy
ImageMedium = Profesional_Army_48.png
ImageLarge = Profesional_Army.png
Modifier0 = $FM_PoolChange_ARM_ProfessionalArmy
Modifier1 = $FM_ConscriptsOfficersPool_ARM_ProfessionalArmy
Modifier2 = $FM_MobilizationPool_ARM_ProfessionalArmy
Modifier3 = $FM_Maintenance_ARM_ProfessionalArmy
Modifier4 = $FM_QualityChange_ARM_ProfessionalArmy
See the last modifier?
Now check
\VGN\GameData\FacMods\109-FM_QualityChange_ARM_ProfessionalArmy.fmd
UID = 109
Name = QualityChange_ARM_ProfessionalArmy
Alias = FM_QualityChange_ARM_ProfessionalArmy
Name = $FM_NAM_QualityChange_ARM_ProfessionalArmy
Text = $FM_TXT_QualityChange_ARM_ProfessionalArmy
ImageSmall = Img_Sml_QualityChange_ARM_ProfessionalArmy
ImageMedium = Img_Md_QualityChange_ARM_ProfessionalArmy
Modifier0 = $fmdXpBonusLandBuild|10
Modifier1 = $fmdXpBonusNavalBuild|3
Modifier2 = $fmdXpBonusAirBuild|5
And your answer is ... 10 free XP for a land unit of a professional army.