Realistic time frame for GL

really depends which GL, what progress you already have, And any other limiting factors (capital ship requirements, kyro income etc).

 If you go for SLKR or Rey.

assume around 20 days per character to gear and relic. If you have them at 7*, 6-9 months is fair for characters.  
finalizer and raddus drop 20 shards per month and they need 5*, so thats around 7 months from zero.

just work towards one of them.

Depends on GL, where your current requirements are at, and your resources. I tend to alternate between GLs that have older characters and newer characters to help alleviate the gear bottleneck, since many newer characters need a lot of kyros and older ones don't. Newer players may not know which those are, though. 

If you're wondering about the newest GL which isn't released yet, I unlocked Leia & Jabba within a couple of weeks of their journey releases. For both my strategy was the following. On announcement, stop spending any resources (except for currencies that might hit max if i don't). This is usually a summer announcement. As the new character marquees release I gear them as far as possible without spending crystals. As they hit nodes, I will usually speed farm them (refresh energy 3x daily, and refresh the node those times if needed). By the time the last of the new characters release to nodes I speed farm them and stop speed farming the others (as they should be x/100), depending on where they are at. If needed, I spend crystals on the last couple of required character shards to help them reach 7* faster if it seems they won't be there when the GL releases. This also depends if I have crystals, as I may need to use them for gear purchases. When all is said and done with this process, it's December. Then I need the special currency to unlock the battles - which is you speed farm low tiers of LS/DS you can get about 75 tokens every 3 days, if i recall, with 75 a week from crystals if you need to buy them. Assuming you win all battles the first time, maybe 2-3 weeks here. Then there's gearing up the GL and getting their ult. which is maybe another week or so.

Newest GL: 

• Announcement in June-July
• Requirements farmed by December sometime
• GL unlocked end of December, early January
• Ultimate unlocked early to mid January

Older GL. Similar strategy, except those characters aren't marquee so I map out who to gear up first. My decisions for that often go in this order:

• Which of these characters do I currently use most? 
  • If they're already actively on teams I use, or have omicrons, then they're worth gearing first so they enhance that team while I work on the rest
•  Which of these characters is a pilot?
  • Even if I'm not using the character on a team, enhancing them enhances my fleet
• Which of these characters will fill a team gap? 
  • If someone rounds out as a 5th or 3rd member to a team, that is considered here
• Highest relic requirements
  • If the final characters needed are high relic, then it feels like a slow burn. So I get them out of the way here. When the final character is relic 3, it feels a lot simpler of a task to complete

The older GLs probably take 2-4 months with the assumptions of:

• You already have all of the required characters at 7 star, and some may already be geared up
• You didn't just use all of your bottleneck gear (kyros, relic mats, or g12 gear) on another special character farm
• You save all of your gear, crystals, etc focusing on this and nothing else
• None or few LS/DS tokens prior to journey

Since you mention being willing to spend some real money from time to time, it will probably shorten your time a bit for already released GLs. If you're going after the unreleased GL then spending money won't do much (i don't think) until all of the required characters are released. But add more time if you need to 7 star one or more characters still (or the ship requirement), and if you are fresh off of another farm, especially if it demands the same gear as the last (i.e. kyros).