What cities would you say have "long summers"?

I arbitrarily define summer as the months with 24 hr means over 20 C. I would say a climate with 5+ such months has a "long summer". Fewer than three months is a short summer. So, Jacksonville, Brisbane, etc. have long summers. Toronto and Canberra have short summers. Again, this is entirely arbitrary. A lot of people would set the summer threshold higher, maybe 22 C or even 25 C.

I agree with the daylight criterion. Despite the heat there, the Philippines didn't really have a summery feeling with sunsets at 5:30 PM and less than 12 hours day length.

I define summer/winter length as the amount of time temps are close to the warmest/coolest plateaus.

So somewhere like Great Falls that has a cool June then spikes in July and August has a brief summer.

Miami has 4 months with highs within 2 f of the hottest average high so it has a long summer.

For winter, Boise has a short winter as February is significantly warmer than December and January.

Shitchester doesn’t warm up much in March and it’s still cold and snowy so it has a long winter.

My arbitrary definition of a summer month is having daily means above 65F (18C), and day length really does not matter. To me, tropical climates are defined by the presence of a perpetual summer. The traditional 4 seasons shouldn't apply to tropical climates however, for tropical climates are much more defined by rainfall amounts and patterns than by temperatures. Thus, your typical SE US subtropical climate, such as Houston or Charleston, would be a textbook example for long summers.

Not exactly related, but Myrtle Beach is a full-on 4 season climate. Winter just so happens to be the ugliest, especially when boomers harp on about warm season grasses which can take the full sun caused by the extremely open areas cause boomers hate trees that much. Fescues work well with part-shade and can do in the Deep South, plenty of examples, especially in the Piedmont like Atlanta and Greenville, South Carolina. 

"Long summers" would be like 72°F average high for April onwards to October, and that probably means October would be 75°F+ for its highs, but if the lows are below 53°F, then it's not as long. 
It's a very subjective matter that really counts on how it actually benefits you, and for some people, they have no choice. People get born in climates they don't like, which is the primary reason why they think their summers are "too long" 

Cities that I would say have "long summers" are like zone 9 cities like Houston, Jacksonville, Orlando, and Savannah, GA. Zone 8 cities typically have some setback of not totally being summery for most of the year, which is why I pick zone 9 as the starter zone for that. However, that cannot apply to the west coast, mainly due to the fact that places like Seattle and Portland, OR have 4 seasons, but also, places like Eureka CA really just have 2. That is to say that Eureka, CA has more springy temps for its summers, but every place on earth except equatorial climates have summers pretty much.

I worked in Brisbane between May 1997 and Feb 1998. I thought the winters were plenty cold enough...lol. The boss had put us up in flat in Newfarm ( almost in the City ) without any heating and the nights commonly dropped below 10c during winter. The kitchen had a fan forced oven, so that became the kitchen/living room heater....haha.

Yep, that seems cool enough for me. Temperature averages make it seem like a place is warmer than it is when it isn't. Sydney's winters look pretty mild on paper (climate statistics), but they can be pretty cold, especially in the morning and night. 

I would assume that, like Sydney, inland Brisbane (by the hills) is much colder at night, no?

Cities that have noticeably longer "warm seasons" than "cold seasons" on WeatherSpark. Mediterranean climates (at least in the US) tend to have longer "winters" and shorter "summers".

My arbitrary definition is having average highs of 80 F (26.7 C) or above for more than three months, or means above 70 F (21.1 C) for three months or more. Somewhere like Omaha, Nebraska would be the northern limit of this.