Grind

Intro

Completing all the quests and M1, M2, and M3 in Aiort Graveyard gets you to level 95.
The next set of quests is available at level 105.
There may be some miscellaneous quests available before 105.

M5 Runs

The M5 run is not a full mission, but just the beginning part.
It should take between 5 and 10 minutes to complete plus a couple of minutes to restart.

Level XP per run Estimated number of runs Total time (high end)
95 2.3% 44 About 9 hours
96 2.0% 50 About 10 hours
97
98
99
100
101
102
103
104
105

The [Spirit of the Liberators] Cash Shop items gives 30% more XP
It currently costs 7 coins for a 1 day item and up to 152 coins for a 90 day item
New accounts get a free 7-day Spirit of the Liberators

The [Great Commander's Baton] gives +20% to the user and +10% to the Group
New accounts get a free 7-day Great Commander's Baton
The Baton is currently not available in the Cash Shop

Full M4 Runs

Altar of Darkness is the shortest mission available in the Chapter 5.
According to the Japanese wiki, doing the full M4 mission is more efficient than doing M5 runs
It takes approximately 108 hours to level from 92 to 105 doing full M4s.
It takes approximately 144 hours to level from 92 to 105 doing M5 runs.

Full M5 Runs

It takes approximately 66 hours to level from 92 to 105 doing full M5 runs.
However, you need to have some 100+ players in order to complete M5.
The recommended level starts at 105 and the end boss is level 130.

M1, M2, M3, Tower of Wind

Due to the length of these missions, it more efficient to do M5 runs, full M4 runs, or full M5 runs.

Coliseum

Level XP per run Estimated number of runs
94
95
96
97
98
99
100
101
102
103
104
105

Sources:
Leveling in SUN (Listing and timeline) posted by Arceb for grind time estimates from Japanese wiki